The 3.7's impedance drops to less than 3 Ohms. Thiel recommends 100-600 Watts. Jim Thiel once told me that his power recommendations were based on the 8 Ohm power rating of ss amps capable of doubling down. He also said that the small impedance range of his speakers worked well with tubes so long as they had enough power to compensate for their typical inability to double down, i.e., you might need double or more of 8 Ohm tube power depending on the chosen speakers compared to ss power. I would suggest that you would need about 300 Watts per channel of 8 Ohm tube power to compare with a typical high quality 100 Watt ss amp. In that I think the old adage of doubling the manufacturers minimum power recommendation has some merit, in my opinion 600 Watts per channel of tube amplification would be even better.
