Energy lists the impedance as 8 ohms nominal, but notes that it may dip down to 4 ohms. Without more information from the manufacturer (like an impedance curve) it would be difficult to say whether it is tube friendly. There are professional reviews on the Energy website, though unfortunately none includes lab results.
What to do? Well there is anecdotal evidence: you can peruse the forums here to see if anyone else is using tube gear with the speakers (you'd want to pay attention to what kind of tube gear - sort of an "apples-to-apples" comparison to the amps you're interested in). Also, the
SoundStage! review includes some information that might be helpful, for example:
The matching of a pair of loudspeakers to an amplifier is critical for achieving the desired sound. My Graaf Venticinque is a 25Wpc push-pull tube design running in pure class A. It is not an especially tubey-sounding amp in the sense of having a great deal of bloom in the low end, but it is still, electrically speaking, a tube amp. Although rated into 4 ohms, it will not drive all loads evenly. My Amphion speakers are exceptionally tube friendly, with an impedance that never drops below 7 ohms. In general, I felt that the RC-10s were very compatible with this amplifier, but I do suspect that a solid-state design, even of moderate power output, would have taken tighter control of the woofer and produced a more articulate bass.
Hope that helps.
