Look at the schematic. Plate voltage on V5 is 111/106 volts nominal which is the higher potential seen by the .1 coupling caps,
and there's no scenario in which they're going to take more than that, so literally there isn't any reason to suspect that a 250 volt rated cap is not sufficent.
I did find my stash of 630 volt .1 capacitors and if there is any need to go back into the amp I'll replace those .1s with 630 volt rated ones simply because...why not? but I don't see any reason to think that it's a matter of compelling importance. I am not saying to lower the voltage spec, but I am saying that the spec based on the original installed components seems to be higher than is justified by any observational evidence including, of course, circuit voltage readings.
Truth is, those orange drop caps in this amp are getting close to 40 years old and I've seen more orange drops failing over the past few years,
as they age, even though in every case their voltage rating was substantially in excess of the voltage they were working at in circuit. So I'm just going to attribute the issue to age-related degradation.
So far, I have yet to see a Panasonic film cap fail in a circuit that was below the cap's voltage rating. Since the circuit in question is under 120 volts DC, there's no reason to think that a 250 volt rated cap is underrated for the task at hand. Literally no reason.
I never advocate dropping voltage ratings on components but I've never seen such overrated components installed in a lower voltage circuit before, either.
So why was Mesa installing 400 volt parts in a circuit that runs under 120 volts? Could be a number of reasons, but reducing the number of parts to stock could be the answer.
You can make arguments for why I should not use 250 volt rated parts in a 120 volt circuit when the original parts were 400 volt rated, but you can't make an argument for that which is logically supportable.
Fact is, it's a circuit that doesn't exceed 120 volts. If a capacitor rated at 400 volts failed in that circuit, it wasn't due to excessive voltage.