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

Takman resistors, film bypass caps on power supply electrolytics. Resistors have flavor, bypass caps tightened up bass.

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.

Well, there are external and external changes to consider. Much of above are good suggestions, I'd add ball bearing type footers, they always add air and spaciousness, however, may change timbre to some extent. But then timbre changes are concern with every isolation device, and I've tried every one of common designs, hanging your equipment from ceiling best, but of course that's silly suggestion in this instance.
In the end you could try every external change possible and may find same issues remain, and this is quite possible. Resistors, capacitors, diodes, voltage regulators, internal wiring, IEC receptacles, fuses, binding posts, I may be missing something. I've done everyone of these mods at one time or another, biggest bang for buck in general terms are capacitors, I'll put the rest in similar category, although some may have more effect than others in certain situations.
You're trying to gain more spaciousness, air. I've been in same position at another point in time. I'd start with coupling caps, may want to try something with silver, or combo silver/copper, Mundorf or Duelund here. This will most likely add exactly what you need on top. Resistors may be next move you'd make, depending on what QS is presently using, he may be using carbon comps, nice warm resistors, may be part of dark issue. Everything else would be of secondary importance.
I pretty much guarantee contact with OEM will not go well, they'll be offended you want to change what they consider optimal implementation of their design, you're trying to alter their child. I know this from experience, and this even if they later adopt your modification!

Also, do only one change at a time, do the caps, nothing else, listen for at least 200 hours, only make further changes based on what you're hearing at this point. You may not have to make any further changes.
Always remember that every single one of these changes may alter sound quality in ways you may not like. Change may also help in one area, not in another, so many parameters of sound to think about! For instance, you may get more air, spaciousness with certain capacitor, but it may alter overall timbre in ways you may not like. Changing out parts is like food recipe, a little of this or that may make all the difference, you need to be prepared to alter things as you go along.
All my warnings are just things to be aware of, parts changes can take a certain piece of equipment from temporary to permanent solution.

Do grannyring's suggestions, these may be end game pieces for you then. While I've not yet tried the ODAM, I will drop them in my phono pre in a bit. Reportedly right up there in sound quality with best, and their relatively smaller size vs. film caps makes them better, easier fit. Other parts he mentioned I've heard or modded with, all good. I'm using the Airlock for all my diy power cords, better than Duelund here.

I'm using combo of silver litz and Duelund copper internal wire, Duelund silver bypass cap on power supply in my custom 300b monoblocks. Flavor wise, wonderful timbre, extension on top, controlled bass. I know the silver internal wire and Duelund silver bypass has much to do with attainment of this sound quality. I'd presume the ODAM with Airlock silver would be avenue for you.


I'm not sure all techs will take on custom work, many frown on customer spec'd parts substitution. Don't want the liability if you're not happy with sound and/or don't trust you're substitutions are correct for circuit.

Just came across post from Aric of Aric Audio. States he prefers the Miflex to ODAM in his amps and pre's. Just goes to show you're going to find all sorts of preferences in caps and most other boutique parts.

I like the Miflex, Audyn True Copper Max, VH Audio cutf, Mundorf Evo, Jupiter copper film, Duelund copper, these all among the best, again, sure odam right up there. Personally, I wouldn't sweat it, get any of these, your amp and pre will be so much better.

Don't know if you're aware, but Humble homemade has reviews on most top line caps, will give you some idea of sound qualities.
I don't think you can go wrong with any of the top rated copper films, the Mundorf Evo might work if you didn't go with silver internal wiring. I'm just thinking you're looking for spaciousness, more extension on top, reason for silver. You also may not need any silver, that's up to you.

Good for you!  Only personal experience will allow you to determine whether benefit worth the cost. I expect you'll find the benefit well worth the cost using any of the top flight caps. And aren't all moves in audio taking a chance, purchasing new equipment is as much, perhaps a greater risk. Too many allow fear of failure to interfere with progress.