Power has nothing to do with your question as long as you're not power limited, and with 90W, you're not. You don't need matched power handling near as much as timbre matching with your mains, and most especially, clear dialog from a center channel.
It seems obvious that a 3rd ONS TM is the leading choice for a matching center... if ONS TM's meet you needs across the front. That's the bigger question I have from this and the thread you referenced - where are you going? What's the goal?
What I'd like to suggest, especially if the engineering aspects appeal to you, is a simple, basic set of speakers that will keep you entertained, but which will also reveal some degree of shortcomings. The idea is to make you aware of the shortcomings at the same time you're learning more about audio equipment, loudspeakers and rooms. Then you can make a conscious tradeoff among price, performance and capability.
If you want to learn to design speakers, get either Alden's or Dickason's books on loudspeaker design. If you want to understand audio, with a focus on speaker performance requirements rather speaker design, I highly recommend Floyd Toole's book, "Sound reproduction, loudspeakers and rooms." You can build anyone's design, but you will be designing your listening space yourself.
I suggest this as a starting point because you have a fully untreated room, and some geometric constraints that pose challenges. Some approaches to those challenges work better than others, and as an engineer, you want to know why one's better than another. Toole tells you why. Plus, no speaker can sound good if the room sounds bad...
There are two things you can do differently today that will help the sound.
You said: "They put out some good bass too which I wasn't expecting
That may be a result of placement against a wall. Per Paul...
"...but since they have very full bass (or as some might say "full BSC"), they sounded best put up on a little stand...."
BSC is baffle step compensation, and it's an engineering term you'll want to learn. There are several on-line tutorials. The idea is that the spatial distribution of sonic energy depends on the wavelength compared with the size of the baffle containing the speaker. An "infinite baffle" is an infinite plane with a speaker in the middle - front and back waves never meet. Each (front and back wave) radiates into a hemisphere, a 2-pi steradian solid angle.
As baffles become practical sized (and back waves trapped in boxes), at wavelengths long compared with the size of the baffle, the baffle has no effect and the driver radiates into the entire sphere, a 4-pi solid angle. As the frequency rises, wavelength shortens until it's small compared with the baffle width, and we return to 2-pi radiation.
The transition from 4-pi radiation to 2-pi radiation is called "baffle step" and BSC the compensation for it. It's easy to see that you halve the radiated power density when you double the solid angle. Energy conservation is easy to see. However, diffraction provides a complete description, including intermediate wavelength behavior and appropriate frequency limits for a given baffle size.
Key message: you can greatly change a speaker's bass/low-midrange response in the entire room by changing the speaker's distance to adjacent walls, especially the back wall, and this is an XO design variable. An in-wall speaker design will sound very thin if used free-standing, while a free-standing design will be boomy if it's against the wall. Since Paul thinks ONS sound best away from walls on stands, try different placements/wall distances and listen for differences. I'm sensitive to tonal balance so this effect jumps right out at me.
The second is to get listener's ears away from the walls - your couch is too close to the wall.
Walls limit the motion of air molecules, making them displacement minima, and so pressure maxima, and we hear pressure. Try it yourself; move your ear closer to the wall when playing fairly loud music and see if the lows aren't reinforced the closer you get to the wall. Again, the issue is tonal balance, but this time, for each listener.
That's it for today's "experiments in home theater." Next, we should talk about room treatments; those broad flat walls will enhance some aspects of your listening experience, especially in stereo. However, reducing front wall reflections is a consensus winner for all applications. Not every build has to be at this level, either.http://techtalk.parts-express.com/sh...d.php?t=227439