Hpower, I think it would be best to take your amp to the dealer and try it. If they're not familiar with tubes, they'd probably want to hear one themselves. Make sure they have the speakers hooked up first also, since they may not know tubes, or tube amps. I don't think it will quite have enough power myself, but that would be easier than taking the speakers home IMO. I never owned that amp myself, but I'm familiar with a lot of tubes/tube amps, and Magneplaner speakers. A 300b puts out about 8 watts, driven comfortably . Higher outputs shorten tube life. With this amp putting out 21 watts from a pair, I think it will be overworking trying to drive this type of speaker, with a pair of 300b tubes, per channel.