I asked Wendell at Magnepan that same question and he said:
"If the amp doubles its power between 8 and 4 ohms you will be fine." In my experience, the Maggie .7 sounds best with class-A solid-state or a good push-pull tube amp.
@invalid First, when I took in my Krell for rebuild I hooked up a few integrated amps and receivers that I had on hand just to see how they would sound. It was unmistakable that these amps produced a different frequency response than my Krell. Specifically the bass was reduced in volume and "anemic." I extrapolate that a tube amp would produce the same result.
Second, there is a very good "Thiel Owners" thread on Audiogon where this phenomenon has been discussed. I partly based my statement on real world experience of several Agon members.
Third, I've been to three different audio shows where they had the larger Wilson speakers. In every case they were driven by D'Agostino amps that double their power down to 1 ohm. I've read many times in reviews and forums that the larger Wilsons sound their best with a high current amp. Reviewers have also noted that when they drive a speaker with a difficult impedance curve with a tube amp the bass is often weak. Check out the review of the Octave Audio Jubilee tube amp in the September Stereophile. JV Serinus describes this exact phenomenon with his Alexia II speakers. Then, in the testing section John Atkinson praises the amp for having lots of power into 8 ohms but he doesn't recommend it for use with speakers where the impedance drops below 4 ohms. I've read similar examples dozens of times. Over and over again reviewers state that the tube amps they are reviewing are not appropriate for speakers with difficult impedance loads.
I can't believe there is even a debate about this. It is well known that driving a speaker that has a variable impedance curve that dips below 4 ohms with an amp that cannot produce the appropriate current at the low impedances will change the frequency response of the speaker. It's a well documented fact. If you can produce evidence to the contrary please do so.
If the amplifier is able to act as a voltage source at less than full power then the contray is certainly possible. For example the Osiris amplifier that @akg_ca mentioned can't double power at full power as load impedance is halved, but at lesser powers it can. In a case like that when the current demand is exceeded, rather than changing the FR the amp will simply clip.