What would you do differently next time?

Do it right the first time. If you have to wait to get the build you want, do it. You will end up adding it later. Have no regrets.

With advances in sub technologies there is no such thing as 'doing it right the first time'..if you love this stuff..you always on the go.. some of us are restless and curious with an insatiable desire for the ultimate sub... just ask The Ear....

I have to agree with Kanaris. Most of us try to do it right the first time but this little bug called DIY bites you and you get addicted. SO after you complete your first build you are itching for another fix ; a bigger sub , more power , a different alignment , etc.

In regards to building it differently next time.... I would probably try a veneer finish, although I love paint.

Go order more than what you need of everything, maybe even a spare driver for an 'oops'. I really wish I had ordered another 18" TC LMS 5400 when they were readily available.

Research research, research as much as you can. I chose to go with QSC as my amp of choice, based upon many many hours of reading. Do it right the first time, overkill is OK, but you still need to occasionally move your creation.

Making a 3" thick box is way overkill. even for 4 18" PR's, and two 18" active drivers. Mine is 1.5 with many large holes in it, and is still hella heavy.

With regards to bracing, the KISS principle is good here. A piece of lumber, ply or MDF say 3/4 x 1.5 glued on each large panel will do almost as good as a window pane style brace, and requires no extra effort, I have used scraps in the boxes I have built.

Double baffle is OK, but generally not required.

It'd be easy for me to agree with 'do it right the first time'. There's a lot of logic in that, especially for me, since I have upgraded every single speaker system I've built except for the first one (c. 1988), with better components later. That said, most of them I didn't upgrade until several years later, and the original drivers were so inexpensive, that I'm really not out much. So I don't have any regrets. I thoroughly enjoyed all the interim or version 1 of all my speakers for many years, and I love the huge bang-for-the-buck. The updated ones are certainly better, and hey, whose garage doesn't need a dozen or more spare woofers, miscellaneous tweeters, and enough crossover parts to choke a dinosaur.

With subwoofers, it's pretty tough to say get it right the first time, given the way that Adire, Dayton, and countless other companies kept pushing the performance envelope ever larger. At least they're improving at a much slower pace than say, computer processors. So while you can build your cabinet right, and it will stay right until it rots away - not so much with the woofers.

If someone researches enough online (this site and the other DIY builds sites), if a person asks enough silly questions, if a person models their driver to know hypothetical performance and if a person takes their time. There simply no reason to screw up a sub build, after all its just a solid overbuilt box

Key points.....

1. Make sure the driver is modelled (including what Watts you want to give it, port velocity, cone excursion....know your driver!!)
2. Make sure you read other builds first to understand what they did.
3. Even follow someone else's build because all the great ones have been done (NeoDan has the best instructions out there!!!)
4. Overbuild your box if you are unsure!!
5. Worry about the finish AFTER you have a well performing sub!!
6. Measure 5 TIMES, cut 1 time (or for me its always cut twice )

What would I like to do the NEXT time...add even more subs. Yes I will not be outdisplaced.

Seriously have the time to finish the boxes I have built,round the corners and sand them. That is all

Always measure several times,recalculate if in any doubt. All parts must fit, if they do not something was botched up along the way.

I built a sub a couple years ago, but due to some changes in my equipment layout, I'm rebuilding the box (and also doubling it from two to four drivers). Like many, I use the glue and screw construction method. This time around I used PL Premium Polyurethane Construction Adhesive and Spax #6 1.5" MDF screws. These combined made construction much easier over the previous build. This glue really does bridge and fill gaps very well (as opposed to regular polyurethane like gorrilla glue that gets foamy and isn't really that strong in gaps, and regular yellow glue that doesn't really bridge gaps very well at all), it's much thicker so it doesn't run like the others, it's very strong, and it's cheap (I think around $4 for a caulk sized tube). And the Spax screws really do work without splitting when screwing near the edge, and even self-countersink. They say they don't need pre-drilling, but I pre-drilled anyway. MUCH better than the drywall screws I used last time (which still split some even with pre-drilling, and had to have counter-sinks pre-drilled).

Darin: I take it you shop at Lee Valley? Seriously, I have never had any luck at all with screws in MDF, even pre-drilling, maybe i'll give the Spax ones a go-around.

I also have used the construction adhesive for building sonotubes. One thing is that it can be a pain to remove any excess, unless you use a sharp knife.

I'm not familiar with Lee Valley... maybe there's a joke in there somewhere that is over my head? On the glue, I've been wiping it flush on the outside seams, but haven't yet done any filling/sanding. Hopefully it won't be a big deal. On the inside, I've just let it spooge where it wants to. I was pretty skeptical on the spax screws, but I swear I haven't had a single split or even bulge. I started off with manual countersinks and pretty liberal pre-drills (since the screws only need to hold strong enough to clamp the glue), but then I accidently broke off the drill bit, and had to go down one bit size for the remainder of the holes. I was surprised to see that even with the smaller holes, there was no splitting. I think the key was sticking to the #6 screws. To go up in length past the 1.5" screws required going to #8 screws (at least with what Home Depot had in stock), so I stuck with the shorter and thinner screws. I'm doing standard butt-fitting with 3/4" MDF, but even with the screws going in roughly 3/8" from the edge of the surface board, and into the edge of the other, no problems in either one. I'm still afraid to trust them on the "no predrilling necessary" claim though.

No joke on the lee valley, that's the only place I have seen the 'spax' screws advertised...

Lee Valley is a great store. I've bought a lot of things from them, including some great hand planes.
FWIW McFeely's has spax screws.