I think all HTiB nowadays using class-D amplifiers. Panasonic is using a TI chip and Sony is using their S-Master technology. The older Denon HTiB was using some Tripath that sounded really great.

I am using a Panasonic XR55 AVR (here's a review at AVS forum where it is used in a stereo setting)

Of the newer AVRs... Pioneer is using ICEPower (Bang & Olufsen technology) on the high-end SC-LX90 and a couple other models

Sadly Sony has stopped making class-D AVRs (last I think was the DA 9000ES)

If you are really interested in class-D ... this article explains very neatly how it works, then check out the old PWM amplifier list I made at AVS (fairly recently updated)