While my new 4 Pis are slowly progressing, I've been listening to my Theater 4s with a new kit amp I've built: the Amp 6 Basic from 41Hz.com. This is a class D amp based on the TDA 2020 chip and produces about 12 watts/channel into 4 ohms, less into 8 ohms. I have to say, this little amp about the size of a deck of cards is one of the best amps I've EVER heard! While not quite as visceral and 3D as my tube amps, the clarity, and musical flow is astounding. And the Bass? -Wow! And unlike many low power amps, these do ALL kinds of music beautifully: from classical guitar, 50's jazz vocals, full on orchestral, to head banging R&R. Expressive is the word that come immediately to mind.
You guys who like DIY just have to try one of these! The kit is dirt cheap (thirty something) and just needs a 12V power supply. I've used the 2 amp regulated supply sold by Parts Express which sounds great (no hum) but am thinking that a 75 watt switch mode P.S.borrowed from work sounds even better though a bit more money. Still, the total cost is WELL under $100 and the sound rivals anything I've heard in my system. Of course, my Cambridge Audio 640C V2 with new opamps and Black Gate caps swapped in has had some positive impact as well. But the amp really is good with the Pi's and several would make a great cost effective multi-amp system: good highs and deep, well defined bass. Huge soundstage for those that like that.
I know that this sounds like an ad but I'm really jazzed about the sound of my system these days and wanted to share!

Happy Spring to all!

Garland