I have a Darkglass Alpha Omega 500, but I also have a Vintage Microtubes Pedal, and I've played the Microtubes 500. I don't play metal or anything, and I really like my Darkglass stuff. The Darkglass stuff is peculiar in that it has a strong, tight but uncolored clean sound which is generally blended with parallel overdrive or distortion.
Even the low-gain overdrive sounds can sound slightly grainy on their own, but they mix with the clean sound of the amp to provide a lot of clarity. It'll never sound like a V4 or an SVT in that the overdrive sound is seldom entirely cooked. It's always a tiny bit separate from the clean sound. Even with blend maxxed it sounds like it was designed to let some low end cleans through. By contrast, I tried a a Markbass Little Mark Vintage that sounded a lot more like a regular amp. I imagine a TH500 also has a more integrated sound.
That said, the parallel dirt, whether on the Microtubes or Alpha/Omega, sounds amazing in a live mix because it'll give you the dirt you want along with the low-end clarity you want. The Microtubes engine does a decent job of emulating classic Ampeg sounds, and the Darkglass can go a lot further into distortion.
I wouldn't worry too much about the Class D aspect. Your Hartke was similarly newfangled in its day. The technology has been around for a decade or so at this point.