I am driving Gallo Ref3 speakers (88dB sensitivity) with a pair of C-J Premier 12 monoblocks. They provide PLENTY of power, even for loud rock music. I am not familiar with the Opera Callas monitors. But there is more to efficiency than the sensitivity rating of the speakers. If your speakers present a difficult impedance load, a tube amp may have trouble providing adequate current. The C-J 12's are powerful and beautiful-sounding amps. But with your emphasis on power, I suspect you'll be happier with a high-current solid-state or class D amp.