Karls-I haven't tried the zh270 into a load that dips low or in your case ruler flat at 3.5 ohms but have read and heard many conflicting comments on its performance at the frequency extremes into such loads that I don't know what to think. How about you Twl? Nah you're definitely the wrong guy to ask :)
I sure am curious myself and anyone in the Central Florida area that has such speakers I will gladly bring over this amp to find out first hand. Everyone I know has medium to high efficiency speakers and the amp sounds great on all of them. The high frequency impedance converter adjusts for impedance dips unlike traditional OTL amps which just run out of current unless there are enough output tubes to provide enough. Transformer coupled amps also do the brute force gig which is why they don't fare well into such loads, especially in the bass and why ss is a better choice for across the board performance.
Regardless, my guess is that the tonal balance especially at the frequency extremes may be affected with certain loads. With my speakers the amp is well balanced and extended across the frequency spectrum. The high frequency performance is also right up there with an absolute crystaline clarity with air and space to match. I really can't hear any problems anywhere but I'm still looking. You know something is on the ball when you start critically listening and find yourself just enjoying in spite of yourself, it happens all the time :)
I sure am curious myself and anyone in the Central Florida area that has such speakers I will gladly bring over this amp to find out first hand. Everyone I know has medium to high efficiency speakers and the amp sounds great on all of them. The high frequency impedance converter adjusts for impedance dips unlike traditional OTL amps which just run out of current unless there are enough output tubes to provide enough. Transformer coupled amps also do the brute force gig which is why they don't fare well into such loads, especially in the bass and why ss is a better choice for across the board performance.
Regardless, my guess is that the tonal balance especially at the frequency extremes may be affected with certain loads. With my speakers the amp is well balanced and extended across the frequency spectrum. The high frequency performance is also right up there with an absolute crystaline clarity with air and space to match. I really can't hear any problems anywhere but I'm still looking. You know something is on the ball when you start critically listening and find yourself just enjoying in spite of yourself, it happens all the time :)