A Shockingly Straightforward Query

Since you can't audition either one then the choice is simple: buy the one you can return no questions asked.

If neither one can be returned then the choice is even easier: keep looking.

an10490413
  Bob Carver 350s or the Theta Prometheus. The selected amps will be driving a pair of Martin Logan 11A speakers.

"MartinLogan doesn’t publish impedance, phase-angle or frequency response curves for this speaker. However, they do state that their impedance dips significantly at higher frequencies. This low impedance, (0.6 ohms at 20KHz) with a corresponding increase in phase-angle suggests that amplifiers that aren’t stable into low impedances shouldn’t be used.
MartinLogan’s online FAQs recommend an amplifiers that doubles in wattage between eight and four ohms and again increases its wattage at two ohms. This would seem to eliminate most amplifiers from consideration."

 This tells you what sort of amp to look for, and it's not tubes, if you want to extract the very best out of these speakers.
But if your happy with "a bit of a compromise" the the Carver tubes will be fine.

Cheers George  

an10490413, for what its worth, I use Pass Labs amplification with my Montis (INT-150).  I'm sure many others will work as well.