The current WP7's don't drop below 3.6ohm either anywhere in the curve (European Review showed that spec) and are amazingly efficient! Also if you wish to drive them with a tube amp, you just change out the included rear ports as documented in the Wilson manual (which both come standard) if your tube amp doesn't have the higher damping characteristics (not a problem with the Lamm 18watt's though) If you still have too much boom in a small room, have Wilson make up a European spec Wilson for you.
My WP7 are outstandingly efficent and I power them with 100watts (Lamm M1.1) and I am unable to stay in the room at less than half gain of my preamp. Also back when the WP 5.1 first came out I listened to them on a the Bat 60watt tube amps and they had no problem powering the Wilson's. I have heard WP7's on VTL 450 monoblock and the less efficient Sophia with a VTL stereo 150 both without strain. Neither speaker was designed for a 300b 5watts!
So I think unless you have tried it, don't knock it. Listen and then Judge for your self.
Both speakers are amazing, the Avalon's Burl finishes are incredible, but then again the automotive paints and finish quality of Wilson is bar none also.
I considered and listened to the Avalons but have thought Avalon bass was more congested and was missing some of the amazingly piston quick Wilson Bass (I play the upright bass and electric bass) and the only other type of speaker which has been able to acurately produce this were large planars (but then dynamics where serverly sacrificed (Didn't listen to Sound Labs as they wouldn't fit in my house)
Also the WP7's can be moved around a lot easier because the 70lb Watt can be removed from the 110lb Puppy. Try Dragging around the Avalon by yourself. If you have a significant other who will help with position then no problem here, but it was easier for me to have my Excellent Wilson Dealer uncrate and position them for and now I'm just tweaking and getting the room right.
Don't get the Wilson's unless you have addressed everything in you system, if it is harsh or dry the Wilson's will reveal it. If your tube preamp doesn't have bass control, you will hear it. But when you spend the time developing system synergy they are amazing (Electrostic imaging with Dynamics!) Also because of the revealing character I visited 3 Wilson dealers in 3 states before I found the right dealer with the right equipment to make me buy. Now I spend more time listening to music instead of tweaking... If you happen to live in Scottsdale, AZ visit Esoteric Audio for your Wilson qwest, and for the Avalon's get to Boulder Colorado to the factory, but don't bother listening to the Wilson dealer setup in Denver (yuk)
Transparent Cables, Lamm, VTL, Rowland, ARC have all provide good synergy in the best Wilson systems that I have listened to.
Classe, Levinson, Aragon, Macintosh, and Bryston, Audioqwest, or any silver cabling have all been too harsh and brittle to be enjoyable.
These are my opinions, if it works for you then enjoy your setup!