240 VAC 60Hz amp running at 230 VAC 50Hz - safe?

Depends on the transformer. 50 Hz is tougher to deal with and often requires derating the transformer if it's not rated for 50/60 Hz. If the transformer is oversized to begin with, that's not much of a problem. However, the problem is compounded by using lower voltage. The real question is whether the transformer will handle "single phase" as opposed to "split phase" without saturation, heat and noise and that's not predictable.

Thanks for the reply Ngjockey.

The response from the manufacturer on this question was

"..the truth is that all of our transformers are manufactured for 50Hz operation. It just so happens that they work just fine at 60Hz in the U.S. also"

Does this ring true?

If it's built for 50 Hz, great. Going the other way can be more difficult.

Let me guess, there's a switch for 120/240V. May be reading between the lines of the manufacturer's response here, but the 240 VAC/60Hz input sounds "legal-eze" for the US export version of the same 230V/50Hz Europe/Asian version without the switch. If so, you're in luck. Why they didn't label it 115/230V 50/60Hz?