Tube Amp, Preamp upgrades -- what did you do that improved your units?

warning ...  as per @tvad's comments

rather treacherous, slippery slope here, with many bodies of regret-laden audiophiles littered about

@tvad  Good points. Regarding (1), if the alteration is not preferred, then I can put the older caps back in, right?  Regarding (2) noted. I think that the QS gear may really be my main rig for at least 5 years and likely much longer. Upgrading it with beneficial effect would solidify the choice. Otherwise, I suppose, I'd be willing to lose some resale value.

@jjss49 Your warning is appreciated. I assume that I might hedge my bets about the risk taken with the modications if I did a simpler upgrade that would be easier to reverse. Because I don't know electrical engineering I don't know where the line between relatively safe vs. risky upgrades are.

@sns Thanks for the list!

The warnings are legitimate, reason you do homework prior to replacement parts. What is the sound quality you're looking for, is there some lacking in present state of equipment. Reading about other's experiences with particular boutique parts will give you idea of particular sound qualities.
And you can always reinstate original parts if you didn't damage them during removal. Not always possible because of issues with getting desoldering utensil where it needs to go, broken legs on removal of original parts happens. Its not risky replacing with boutique parts as long as you know voltage ratings, never replace with lesser voltage or temperature rated parts unless you understand the circuit.
I always figure the labor time and parts you invest in upgrades will never be accounted for, you're not doing this for the financial aspect, solely for sound quality. You may be able to get back what you paid for used items, I recently sold one of my modded amps for same price I paid originally, prior to mods. I've experienced a few buyers over the years attracted to modded units.

Just simply replacing original parts with upgrades is in no way hazardous to the health of unit unless you replaced with parts not up to spec or incorrect installation of those parts. I assume quality control is superior to generic parts, you can even get matched spec parts when requested.

If it does not have remote volume look into fitting a resistor loaded stepped volume pot.

Gold Point used to be recommended, but there are probably other current rec's.

Aside from my inexpensive Bottlehead I heard before/after with a Matantz 7 and it made a huge improvement (especially @ lower/mid volume levels).

DeKay

@hilde45 you may have to go underground over to DIYAudio on this one to find the builders and mod-ers.:)  Not all, yet most here buy and try and rotate new $ amps to find the sound they are searching for instead.  I did high-end Mundorf caps only on my 120 monos. Super duper pleased with the results and would never swap back to the low grade stock caps. Voids warranty and not recommended per Mike at QS of course. Find a competent tech nearby who does this in their sleep if you proceed.