Top-boost switch not working on Vox JMI Conqueror

[seveneves]

It's one of those things that are not strictly necessary, but can prevent unwanted things, sometimes, maybe. Audio preamps like this don't need to respond much above audio, and the excess bandwidth can add noise and support RF oscillation. Not always, but it can happen. One tenet of low-noise design is to not provide more bandwidth than your signal really needs so you attenuate noise outside the frequency band you really care about.
Some amps do oscillate at ultrasonic or RF, and that does eat power, overheat things, and can sometimes cause audible interference as the US/RF gets aliased back down into audio.
The Conqueror probably works fine without it. I like it because it's good design. You might try with and without it to see if it makes any difference to the sound. If not ... leave it in.

Thanks R.G. for the information. I'll be honest, I'm slightly loathe to go back in there now to add the cap. My bad, I should have done so when I switched out C25 from 1uF to 4.7uF and was already in there. As you know, this is not the easiest amp to go into and add/replace components.

Let me ask this: is it safe to assume the amp is not wasting power if the amp measures 32W at output before clipping?

[R.G.]

Sure - it's a precaution only. Unless you have a reason to want to go add it, leave it off.