I've owned two pair of Sound Labs (A-1 and U-1) and over the years ran Air Tight ATM-3, Atma-Sphere MA2, VTL 750, Wolcott Presence 220, Tube Research GT 400 Viva Aurora (both 572 version and the 845 Version) and half a dozen others including some solid state designs.
All of the tube amps I've listed ran the Sound Labs satisfactorily but the Atma-Sphere MA-2 and VTL 750 have superior power delivery, particularly if you're in a large space or play loud. The Wolcott has wonderful control and is VERY reliable.
In terms of tube losses, the most reliable I've owned is the VTL 750, second would be tied between Air Tight and Wolcott and most tube failures came from Atma-Sphere and Tube Research and worst of all the Viva, which I should not have expected to drive an impedance curve like the Sound Lab presents.
Depending on which model Sound Lab you own, the Atma-Sphere may or may not like the load. There are at least four different impedance curves measured from Sound Labs, depending on model, year and design. Very early models rise to nearly 60 ohms in the deep bass and 2.7 ohms in the high frequencies. Oddly enough my old Atma-Sphere MA-2 loved the high impedance and drove the hell out of this difficult design.
There was a time when the midrange in Sound Lab dropped to a very low number (1 or 2 ohms?) and Atma-Sphere amps did not do well with this model.
The VTL 750 will drive any Sound Lab but the sound varies enormously depending on which impedance curve you're dealing with.
The Air Tight can sound thin and lacking in dynamics on these loads and generally does best with NOS Mullard XF-2 EL-34's in the output stage (or) if budget does not allow that, the Tesla JJ E-34L. As you push the power envelope of the Air Tight the high frequencies can get brittle due to stress on the power supply. Again, depending on which model and back plate design of the Sound Lab in question.
The Wolcott Presence 220 has a feedback control to vary the damping which helps a bit on Sound Labs. In my experiments, the Wolcott drives the earlier and middle era design Sound Labs best set to about 10:00 O'clock position on the feedback and selector set to the up position.
The Viva sounds wonderful on low contrast, low dynamic music but quickly runs out of head room at the first sign of difficult program material.
The Audio Research Ref 600 Mk 2 would probably sound wonderful on Sound Labs, I have some listening experience with that amp as well. However, failures of that model are so frequent as to rule them out unless you are a technician and can do repair yourself. Shipping will kill you on these they are so heavy.
Good luck with your project.