What exactly is a digital amp?

TACT, PS Audio, Acoustic Reality, Spectron, Bel Canto, and maybe the Sunfire, are all examples of digital switching amps. Class D and Class T are Digital Amps.I don't know for sure about Class H amps.Class H amps are switching amps, but I don't know for sure if they would be classified as digital amps or not?
They are very efficient and run cool.Some don't even have heat-sinks at all! I have never heard one, but have read rave reviews on some of them, and heard good word-of-mouth on them.

Daltonlanny,
Class H is a digital amp.
I still don't know all the differences between classes higher than D.

Can they really surpass the sound qualities and sonic advantages of top Class A or Class A/AB amps with monster power supplies, huge storage capacitors, and BIG output stages, such as Krell, Levinson, Parasound, Pass, etc? Wouldn't their sustained bass output power, dynamics, and bass extension suffer in comparison to their Class A or even Class A/AB counterparts?

Daltonlanny, I think the TacT is probably the purest expression of a "digital" amp(?). In this case it's not really an amplifier at all, it's really a converter of PCM to PWM, as such it doesn't need those parts to perform. Whether or not it compares to traditional ss technology is still to be answered. I've have only heard a couple of examples in less than ideal circusmstances. One product in a few different rooms with different equipment and an all TacT system. The first one didn't impress me, the TacT impressed the heck out of me. I am reluctant to make recommedations though, as the audition was to brief and compromised. It's interesting that both Rowland and Audio Research are actively pursuing this technology. Audio Research seems to be taking a very conservative approach (is anybody surprised?) and Rowland a more aggressive one. I am very excited by the possibilites with this new technology!

The main benefit of digital amps is bass since lower freequency get substantially less distortion when sampled or clocked. Bass region is the most power demanding region of amplifier and speaker. I know of Tact gear as being pretty smart developed technology allowing the amplifier to be switched onto a bunch of different classes of operation that is controlled by a proccessor which allows to maintain the low distortion parameters throughout the large freequency and dynamic range.
The pure digital amp nowdays maynot approach the advantages of conventional A, A/B classes of operation unless the circuitry sophistication is applied.