Powered speakers are like an albatross around your neck.
If you want more power, you can't. You have to sell the amps *and* the speakers to upgrade.
I have to agree with Bombaywalla. Some manufacturers of ESL's specifically made/make ss amps for use with their speakers, Acustat and Sanders come to mind. Many ESL manufactures use ss amps when demonstrating their speakers. I remember a particularly fine demonstration of Martin Logan's with Threshold amps.
'Many ESL manufacturers'. A bit cumbersome to be a real oxymoron, but nevertheless it is an oxymoronic phrase. And a bit of a red herring, there are not 'many' ESL manufacturers :)...
There is more going on here than meets the eye. ML and Sanders (the Acoustat powered speaker used a tube amp) are both trying to make their speakers work better with transistors by reducing the impedance of their speakers (unfortunately, Quad has been treading this path in recent years too). To this end you encounter some very low impedances (0.5 ohms to 4 ohms) with them. However if you can get around the impedance problem (with a set of ZEROs or the old Atma-Sphere Z Music autoformer) what you find is that the tubes sound better even on those speakers. Sure, such and such a speaker might have sounded 'fine' with a transistor amp, did you then compare it with a tube amp in such a way that that the tube amp was not having a problem with the load? Just because a speaker sounded 'fine' is not the same as it sounding its best.
The old Quad 57s and 63s, as well as most Sound Labs and some Acoustats, have impedance curves that vary from about 2 to 30 ohms or more. Usually if someone with such a speaker and a transistor amp claims that it makes bass, its because the speaker is too close to the wall. They are not winnowing out the performance of the speaker in that fashion.
Paul Speltz, who make the ZEROs, has a letter from Steve McCormick, who makes (or made) the McCormick amplifiers like the DNA1. In the letter, Steve states that while the amp can easily double power into 4 ohms, it actually sounds better driving the 4 ohm load through the ZEROs (meaning that it is seeing 16 ohms).
If you interview solid state amp manufacturers, you will find that they will pretty much agree on this point- just because they can double power into 4 ohms does not mean that they are sounding their best, and you can see that in the specs on any solid state amp- the distortion is higher as the load impedance is decreased.
Essentially, if the goal is the audio *quality*, there is no argument for four ohms or less. If sound *pressure* is the goal, then 4 ohms has a weak argument. Looked at another way, a simple means of making your speaker sound smoother and more detailed is to increase its impedance, not because the speaker will work any differently, but because the amplifier will.
