That's a good question. My first inclination is this: running the transformer at a higher voltage than the rating risks actual harm to the transformer and your amp. Running at a lower voltage "only" affects performance, it isn't a safety issue. But, I suspect the transformers Decware uses would be fine with 10 volts ( < 5%) over the rating so I would be inclined to pick the lower voltage transformer and take my chances with higher actual power. JMO/FWIW and all that.

But probably the best answer would be to ask Steve.

CArlsbad,

I agree with your statements, but only to a point. Maybe this depends on the age of the component, but in my experience, I own a Cayin CD player rated for 220 volts. No problem, I got a step up transformer and used it to feed the CD22. Now, I measured the output voltage from the transformer and it was at 240 volts. After a short time, things went bad and the CD player was repaired. The engineer who worked on it said that the too high voltage caused some trouble in the audio section of the CD player. So, I had two fixes to try and deal with this. One was a PS Audio P300 as you have mentioned to feed the step up transformer. About 105-110 VAC was where I could set it to get the desired voltage. The other option was to use a good Variac, which is what I am currently using. Worked out well. I suspect that the problem is more about incoming voltage variations than anything else, as mine comes in around 120-125 VAC.
As a side note, the PS Audio P300 allows for changing the AC frequency from I think 50 to 120 hz. Very interesting to me with varying results in some equipment. Of course, it gets even more interesting using the multiwave functions that vary different parameters of the AC waveform.

Thanks for all the answers.