Well, I figure I haven't doled out my customary dose of unsolicited opinion in a while, so . . .
Yep, still have the Behringer A500 amps. I'm only using one right now, the small room can get by with 300 or so wpc in the bass.
The big room definitely likes the monos and 500 better. At $200 a piece, no reason not to go monos with short speaker cables if you're inclined that way. Subs under the spheres?
I spent a lot of time with TacT stuff. I had the 2.0s, the 2.2X, and the XP. Those things are spendy, and I didn't like the DACs. So I spent a bunch of money going through DACs to get the sound right. The Altmann and Lessloss were the best I found but something wasn't right. I decided the digital processing itself was the problem and after switching to the Lamm preamp I believe that even stronger. Now this was something like $8k worth of digital XO/processing, presumably about as good as was available a couple years ago.
Now I'm using a complicated XO/processor just for 100 hz down. At these frequencies the processing is invisible as far as I can tell. This works great because these are the problem areas in almost all rooms. And, the processor I'm using is still the dbx 260 which can be had for $500 if you look around. It has all the bells and whistles plus more that I haven't even played with yet (need to do that). Oh yeah, in stereo. Actually it has six channels and the capability to remember 100 presets for movies and such.
It sits in my rack, once I get the room calibrated I set the control screen to 'sub level'. Different moods, volumes, albums want a little more or a little less bass juice. I can spin the dial in .1 db increments to pressurize the room exactly to taste at any time.
You could build this setup for your subs to compliment the spheres perfectly. The subs could be completely passive, no circuitry inside. Park the Behringers behind or to the side and connect with 10 gauge romex wire. Long balanced cables to the dbx which could sit in your rack.
Then of course you'd need a microphone, mic preamp, and Room EQ Wizard. Hog heaven, all DIY, and no passive XO to buy/build/screw up. Even if you got the XO right, it'd still be wrong.
If you designed the subs to sit under the spheres, full phase control would ensure that's the "right spot", wherever "that spot" worked best for sound from the spheres.
I'm tellin' ya, once you get past the active bass learning mode, you Will Never Go Back. I would never again even consider ANY speaker that didn't include this functionality.
