Used to own a pair of 6381's. Usher recommends SS amps for their speakers, but many have had success with tubes. FWIW I tried Nuforce Ref 8 Class D (160w/8ohm) which I believe Usher recommends. I also tried Usher's own Class A, A/B Ref 1.5 (150w/8ohm) and finally a tubed Cary Rocket 88R (30w triode/60w ultralinear). I preferred the Cary in UL over all of them. The Nuforce weren't bad however. The tubes just added that 'tube magic' particularly in mids. Presentation seemed more fleshed out and a bit more forward.
Still, despite my above experiment, I would say the higher the power the better. The Ushers aren't terribly sensitive. Impedance curve, I think, isn't too wild, but in 30w triode mode with the Cary there just wasn't enough there.