Does a 'bright cap' really boost highs?

[maxkracht]

Others have mentioned this, but a low value cathode bypass cap will boost only highs. Say, .22-.68 somewhere around there. You can also cut some volume early by changing the volume pot to 500k or even 250k instead of 1M. A shortcut for that is to add another 1M resistor in parallel with the 1M pot to give you something like a 500k. You might also experiment with just removing the power tube cathode bypass cap altogether for a bit less bass and volume.

[martin manning]

Lastly, you can increase perceived highs by cutting highs in the NFB loop. This is exactly how a 'Presence' control works. Cut highs in the NFB loop, which in turn reduces less gain in the high frequencies, when the NFB is applied.

I like this idea! Note that it is not just perceived highs, in this case you are increasing the gain at high frequencies. Try a 100n or 220n across the 1k5 cathode resistor on the second triode. Put it in series with a 10k linear pot wired as a variable resistor for a presence control.