Dazzdax, living in the UK its very difficult to find dealers with Sound Labs and none to listen to with different tube amps, and as I have had ARC tube amps for many years I thought that the new 210's would be a good match. I was wrong!
They don't have the reserve of output power in the new transformers to keep up with the U1's and kept clipping and the bass response was slow.
After the fact, I contacted a couple of SoundLab dealers in the USA and they suggested several different makes of amps, both SS & Tube, with which they and customers had satisfactory experiences. I don't think either one had tried the 210's with them but both thought it was not a good match.
The Apogees might - or rather should be - more reliable these days with all the improvements and mods that have come along, but back then I couldn't afford the repairs or the Krell amps to drive them.