Most OTL amps are happiest with a fairly high, smooth impedance curve. That being said, in my opinion the MA-1 will work very well with quite a few 8-ohm (and lower) speakers as long as the impedance curve is fairly smooth.
I don't know what the impedance curve of the Alto Utopias looks like, but if it's similar to that of the Nova Utopia then it may well have a 6-to-1 spread above the bass region. We're used to thinking of impedance dips as being the biggest problem, but a peak can be a problem too. Let me explain:
Most speakers are designed on constant-voltage (solid state) amplifiers. If you have an 8-ohm speaker that has a 16-ohm peak in the midrange, chances are it was "voiced" with the expectation that the amplifier will be delivering only 1/2 as much wattage into that peak as into the 8-ohm portion of the spectrum. But the MA-1 is in effect a constant-power amplifier; in other words, it will deliver rouighly the same power into that 16-ohm impedance peak as into the 8-ohm portions of the curve. So as a result, the speaker is about 3 dB louder in that region with the OTL amp than it would have been with a solid state amp. If the peak happens to be around 3 kHz, as with the Nova Utopia, then that 3 dB peak can make the speaker sound too forward and possibly even a bit edgy. Most people will mistakenly blame the amplifier, but actually it's just a poor amplifier-speaker matchup.
Going back to that possible 6-to-1 impedance spread, we could see a 7 dB peak variation in the frequency response of the speaker when going from a solid state amp to an OTL amp.
Now if the impedance curve is very smooth, the tonal balance will be pretty much the same with a wide range of amplifier types. In practice we usually still have impedance peaks in the bass region, which can actually work to our benefit in some cases.
I have some experience driving both 8 and 16 ohm smooth-impedance speakers with OTL amps as well as other types of amps, and in my opinion many amps sound better with a 16-ohm load as long we stay below the onset of clipping. The disadvantage of a 16-ohm load is that a solid state amp will voltage-limit and clip at about half its rated 8-ohm output, and a transformer-coupled tube amp that doesn't have 16-ohm taps will also deliver reduced maximum power into a 16-ohm load (but it may well sound a bit better while doing so).
Duke
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