Most speakers are designed to approach a flat voltage response, not power, and amps to be a voltage source.Agreed, but of course "most" doesn't mean "all." It just means a majority.
If the source impedance of the amp is rather high (tubes) then there will be an HF roll off. The performance of a solid state amp might sound subjectively brighter but that’s not because it’s tilting up the highs, but because it’s not rolling them off.The implication of this statement seems to be that in the case of electrostatic speakers, which are what the thread is addressing, that the tonal balance which results when a solid state amplifier is used is necessarily more correct than the tonal balance that would result with a tube amplifier. If that is what you are saying I and I believe many others would disagree. It's worth noting, for example, that the Quad ESL-57, which is revered to this day by many audiophiles, was designed before solid state amplification existed.