I've played 800 RBs, 700 RBs, 1001 RBs, and briefly had an old 2000 RB (got it used from Guitar Center years ago, but it was on its last legs -- they were nice about taking it back about a week later). The only class D GK head I've played, though, is the little MB200. Owned one a few years ago.
I've owned the Blue Rock (and currently own a Heart Rock 2, its class A/B older brother) and the Aggie TH500.
I don't think that I've played a UL501, but a friend has a PS1000 -- that's my reference point for Epifani amps.
You mention that you'd consider a DB751 -- very far from a class D micro in size and cost -- if not for the shipping premium. You also mention that you're looking for something with a distinct voice (a contrast to the UL501, I presume) that will faithfully relay the tones of your basses and your playing techniques. "Distinct voice" and "faithfully relay" might seem to be at odds, but I think I know what you mean -- something that will communicate your technique -- hand placement, articulation, etc. -- and not make all your basses sound the same, even if it imparts its own color.
All the amps you mention will get a healthy degree of "drive". The Glock has a footswitchable Drive channel (with associated voice control) that will get into full-on distortion if you like. I believe that the GK is similarly footswitchable and that you can get a healthy dose of overdrive with it.
The Glock and the GK will both outclass the Aggie in terms of available power and headroom.
The Aggie... P-bass friendly, a bit wooly and rolled-off up top. Emulates a tube amp in terms of touch-reactive drive and some extra harmonic content, but doesn't have the detail of a good tube amp and doesn't "bounce" like a good tube amp. I had a Mesa Walkabout at the same time and preferred it over the TH500 on both those counts.
The GK amps I've played have a distinctive voice. Somewhat scooped, aggressive in the upper mids. Absolutely will cut through a mix. While I associate my friend's Epifani with a somewhat scooped sound as well, I hear it and the GK amps I've played as having very different personalities. The GKs are aggressive and a bit rough. The Epi is smooth and liquid -- both are articulate, but the Epi bubbles out of the mix while the GK cuts and grinds. Both have EQs, of course, and I don't suggest that they're one-dimensional -- I'm sure that you can cut with the Epi or bubble with the GK, but that's where I hear their natural tendencies.
The Blue Rock, with its EQ disengaged, is supposed to be flat-for-real. Per Helge at Glockenklang, there is no baked-in voicing then. It hits harder than the Aggie and has a lot more available headroom. I had a Magellan 800, overlapping with the Glock, and the Glock hits harder in the lows than the Magellan did (the Magellan has more headroom, more clarity, and more flexibility than the Aggie, but is just a bit compressed in the lows). It doesn't feel like a tube amp, but the Gain control affects feel and harmonic content in a smooth and predictable manner (and as mentioned, the Drive control will take this farther). I don't hear the BR as having the same cutting aggression as the GK heads I've played, but neither do I hear it as having the liquid smoothness of the my friend's Epifani amp. I think that you could get very close with EQ, but that there would still be some difference.
Relative to the Heart Rock 2, the Blue Rock strikes me as a little less extended, top and bottom (whether it actually is or not) and a little less smooth through the mids. Played through the power section of the HR2, the sound is almost identical to the HR2 on its own. Still, the BR is the most solid-feeling class D head I've owned -- comes the closest to the feel of a good class A/B amp, though I don't feel that it's 100% there.
I posted some
clips of the Blue Rock and other amps (not the TH or the GK, unfortunately) awhile back, if you find them helpful.
Miscellaneous: Though I never found it to be an issue when playing out, the TH500 has a loud-ish fan. Slightly annoying for practicing at home, and might be an issue for mic-ed recording. The fan on the Blue Rock is almost silent -- quiet enough, IMO, for miced recording. Don't know what the fan on the GK is like. The Blue Rock is bigger and heavier than the other amps -- close in size to a Mesa Walkabout, but a little lighter.
The GK would offer a definite contrast to your Epifani. It has a reputation for being loud and powerful. I don't know what the used market is like in your country, but it costs less than the Glock and I'd imagine it would be relatively easy to sell if it's not your thing. The Blue Rock would be more a blank canvass, sonically, than the GK. I think the build quality and overall level of refinement would be a step up. It depends on what you're looking for, and what niche you want the amp to fill. The Aggie would be my last choice, personally, but, again, it depends on what you're looking for. The first time I played it out, another bass player I know heard it and tried to buy it from me. I was like, "Dude, I just bought this amp! Let me at least try it out first!" I eventually sold it to him and he's been gigging it and loving it with multiple bands, since. Even though I said it would be my last choice, it would be easier to slot into some mixes (old Motown and R&B come to mind) than the GK.
There are other amps -- people have mentioned the Mesa D800/800+. I mentioned the Magellan (could be a nice alternative to the TH500). Vanderkley amps are supposed to be very nice and very hifi, but even pricier than the Glock stuff. There is the Demeter VTBP 800D, which you might like. You might have a look at the
Handbox R-400 (not class D, but interesting, reasonably priced, and should contrast with your Epifani). The new EA Doubler looks interesting and should be pretty hifi.
In the Vanderkley price range are handbuilt class D amps with tube front ends from MAS/Sonic Farm and Reiner Amplification (there are threads on TB if you care to search)... of the amps you mention, though, I'd be inclined to try the GK or the Glock -- the Aggie if you don't need gobs of power and are looking for something more old-school.
