Thanks. This is why I am asking here, so I can hopefully find a good compromise. My KEFs do have a port on the front by the way, and the examples I gave where just examples for the size / look I am going for.
How low should the speakers go to avoid the "mid-range hole" and at what point the sub can take over? Note that this is going to be just for movies, not music. (if this matters)
When I was intially going through my HT build, I asked my wife what she wanted. She wanted speakers the size of a bottle cap for L/R with the rest hidden. (I asked!) What I did was look at speakers/designs that coered many different sizes from one cubic foot at the smallest and rather large towers I then made a mock up of each speaker with cardbard and tape so my wife could physically see what each size was in the room. She wanted the smallest one of course, but was OK with the larger version (20" H X 12.5" W X 12.7" D) This was fine as long as the center channel and surrounds were hidden AND as long as the finish/grills did not scream "look at me!". I buil the speakers with a dull black finish wit a light grey gril. Anything with a really shiny finish is not a good idea in a dark room as it reflects light which is really annoying. The center channel was built into the furniture and I had to take the furniture apart, put in a new shelf etc. to get it to fit. Done!
My surrounds will be painted to the same color as the wall with white grills to match. On top of the surrounds will be a pretty wood shelf to hold useless fake plants, pictures or whatever.
The concept is to blend them into the wall and confuse the eye as it is drawn to that fake plant (or whatever) The subs are end tables with custom tops she wants (Amazon will provide) in the veneer she wants and has a door on the front which opens so you can see the subwoofer driver. Useless as an "end table" to put things in but you can place things on top. End tables are generally useless anyway, just things to hold junk with so no harm/no foul.
Think of the futility of what you are asking--put it this way. "My wife needs some new lipstick--I should ask a bunch of dooooodes on the internet what I should buy!" The person you should be asking is your wife, she knows more the size, shape and finish options better than a bunc of danglers on the internet!
In reality, speakers are air pumps and the more SPL or bass you want--the bigger the pump has to be--simple physics. The first problem is bass response to match the mains, the mains must reach 80 Hz crossover point without undo stress/distortion at the crossover point to match your single subwoofer (I am assuming you are not running 2 to 4 of them) 80Hz is about the highest you can go without knowing what direction the bass is coming from which will mess up the sound quality--you can go higher but that requires mutliple subs, bass management and so on--probably not what you want.
OK, 80 Hz at a reasonable sound level that won't distort too much at say -10dB of reference (95dB peaks at your listening position) About the smallest I would go is a 5" woofer--and I would go larger if I possibly could. This would be a box at the smallest of at least 6.5 to 7 inches wide. You can get speakers that have say two 4.5 inch woofers and so on but that causes additional issues and expense to do properly. JBL has a center that uses SIX three inch midwoofers and it does go down to 80Hz and that is the level you have to deal with--just physics, don't take it personally.
When you are saying "on wall" I would assume you don't want the speaker to be 8 inches or more deep--you also create issues when putting speakers on a wall--the bass can get boomy and so on. Not much of an issue with room correction, easy to turn things down with EQ but something to know before you go in.
Say your wife can put up with a speaker that is 7" wide X 16" tall X 5 inches deep--then we know what she (or you if you like to blame your wife
) If she can't put up with those size requirements--then you can go inwall. Sure, it is a compromise but wee little speakers that you expect good performance from ARE a compromise! When you remove them, you'll have to repair/paint the wall but that is the price you pay for the design--know that going in.
The other option is to find the absolute largest enclosure you (her) can handle and get them custom made. There are plenty of DIY designs people have made over the years that can be manipulated with the enclosure size to fit your requireements. There ARE issues when doing that, the DIY folks can point those out and don't forget the depth of the woofers (or horns) as there is a natural limit. Any car stereo place can build the cabinets to spec, they can even build the crossovers or the entire speaker in whatever finish you (her) likes. Leather, latex rubber, stone, fur, automotive finishes--the limit is what you desire and how much you are willing to pay. I've seen speakers wrapped in leather, they look tool (to me at least)
Before you jump into the rabbit hole, mock up some boxes that use 5" woofers minimum--those will be the "smallest option". Then mock up 6.5 inch two way boxes, mock up some onwall speakers that use 5" or larger drivers and offer a few options. If she absolutely refuses all of them--get a sound bar, go with Bose or Gallo--throw away any concept of sound that YOU want to hear. You can't get a barrel of performance out of a tea cup--physics won't let you. The best thing you can do is mock up various sizes of speakers--sometimes she won't mind if they are a foot or two tall--as long as they "look narrow" Something that is 18" tall and 7" wide looks narrow--use that to your advantage. If it can't stick out further than the TV for aesthetic reasons, "improve" the TV mount by using one that pivots vertically (nice for internet surfing) and your "new and improved" mount "for HER" also sticks out further from the wall because it has to! This will give you extra depth to work with and "looks right".
Depth was not much of a concern for me--I'm stuck with the "entertainment" cabitnet thing that the TV is placed upon. Her grandpa built it so I modified it (call it modernization) to hold my center channel, the BluRay, TV boxes, drawers to hold DVDs and the like. My wife likes it better now--and I was able to stuff in an 18" tall 54 pound center with light grey grill so hide the thing. I did it for her--of course, it works better for what she wants to do and grandpa would be proud. Everybody wins! Sure, I had to rebuild the furniture, build the center channel to fit but my wife prefers me to suffer with my hobbies--the price I pay.
It is rather simple to get bookshelf speakers to go down to 80Hz to mesh well with your sub--very simple. The issue is when you have demands about size, depth and "pretty". What is "pretty"? To my wife, pretty is stealth--she does not want 7 speakers and a few subs to jump out at you when entering the living room. One of my subs has a stone top on it--very "unsubbish"--most people never identify it as a subwoofer. The end tables are end tables--most people don't open the hinged door to look inside.
Back to pretty, does she want shiny, natural wood venneers and what stain or wood finish? Does she like really shiny finishes, if so, what color? Would a slender on wall speaker that does not stick out further than the TV what she wants? If so, what is the maximum depth? Some people want the speakers to be as wide as the TV for the center or as tall as the TV for the L/R speakers--but fairly shallow.
In reality, you have one shot at doing this right--it would be best to make mock ups of the speakers you want that will go down to 80Hz. Mock up a few different options, she can select which one she likes, she can select whatever finish she wants and you can roll with it. There is no law against changing the finish/veneers or colors of speakers, people do it all the time. Not hard to get the speakers she wants, remove the drivers/crossover board and input terminals and taking it to a furniture place or professional car audio place and they can put whatever you want on the box (it's just a box!) This option can make her very happy but is an extra charge (of course!) She might like that you went to the effort to get a custom made or refinished speaker just for her. My wife (secretly) likes the finishes she selected--her friends told me.
In summation, find on wall speakers that have 5" drivers or larger--mock them up and she can select whatever she likes in the finish she wants. If all your mock ups are not pleasing to her, then you will need to compromise bass response, SPL capabilities and so on until you get there. If you end up with Bose cube size or Bose wanna-bes--then I'd look into a nice soundbar with daul subwoofer options. Not perfect by any means but they are designed specifically to do that so why make your compomises any harder than they need to be?
So get to cutting/folding cardboard--after all, this purchase will be around for a long time so a few hours to find out what size/finish you need will give you the best performance options. Good luck!