Still confused how to power stereo speakers. HT amp or integrated stereo amplifier?

Separate amplifiers tend to sound a little better than integrated receivers, but that also depends on the quality overall. Most seperate systems start getting pricey and the quality is usually high. Typically one would have a preamp and amplifier(s). The A-9555 is an integrate amplifier, meaning it has a preamp built in.

The 9555 is a little bit more expensive than the 606. One thing the 9555 will lack in compared to the 606 is bass management, in case you are using a subwoofer. I don't see a subwoofer output on that model.

What I would do is audition the A-9555 along with a TX-SR606 and see if you can hear the difference. You gain a lot of other features with the 606 (like a radio tuner, and possible use as a 5.1 or 7.1 channel system), so if your ear is happy with it you save $$$ and have a more versatile product. Step up to the 706 and you would have a THX Select2 certified receiver with Audyssey MultEQ auto setup (makes a nice difference in sound I think).