What kind of music do you listen to? I've used both tubes and solid state (and a hybrid integrated) with my Europa's, and each combo had its own strengths and weaknesses.
With tubes, the Europa's really conveyed a beautiful, smooth, sweet sound. Midrange was incredibly pure! But the bass was a little weak in my opinion, and a little on the mushy side. As a disclaimer, however, this might have been a characteristic of the tube amp (Jolida 502A) I was using.
With solid state amplification, the sound has been much more precise and "tight" (for lack of a better word), with crystal clear highs, and much better bass. Bass extends lower, and is noticeably tighter than my experience with tubes.
If bass is a priority to you, I would suggest sticking with solid state. I've used both the Audio Analogue Puccini SE and the Audio Refinement Complete with excellent results. Make no mistake, with the right amp, these speakers are capable of very good bass reproduction. If bass is not too important, I'm with Pete... Tubes really do make some sweet music with the Europa's.
I would e-mail Roy Johnson (the designer and owner of GMA), username Royj, and ask his opinion. He's been very helpful to me in the past with respect to choosing cables and equipment to match with the Europa's. Having said that, I've heard a lot of Blue Circle gear, and I imagine a Blue Circle tube amp would be an excellent match.
Best of luck! You are wise to build a system around such a top-notch set of speakers.
-EWHA
