bpinder, why don't you contact Sonus Faber and see what they say, I don't know how much input Franco has anymore but he still owns a ZH-270 and the first introduction of the Stradivarius was done using the modest ZH-270, I think it was Fremer what visited Franco, look up the stereophile review.
It surprises me that the ZH-270 is strained at higher levels, have you played with the volume control.
There are several reasons why I am surprised but one mostly is that the ZH-270 does not act like most amps when pushed, because of the sophisticated auto bias, it senses when it's close to clipping and reduces power to the amp, this happens at a super fast rate up to thousands of times a second, so all that might be perceived is a slight reduction in dynamics.
One way to tell if you're reached close to output power is keep turning the amp up, it should react with very little difference in perceived volume, if there is a volume increase then it's elsewhere in your system.
These amps are used by so many manufacturers for designing it will surprise you, including the speakers mentioned here by jwmazar Chris Sommovigo's Stereolabs, what makes these amps great for this, is what makes it unforgiving for the average audiophile, they are not filters which is a limit of the typical design constraints, get it all right or this amp is going to tell the story.
