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

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.

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.

@sns  Thanks! It's like speakers -- many good ones out there, can only try so many!

I am looking especially among QS folks, with some eye to what they like in an amp. That said, they'll have different rooms, different speakers, etc. It's a crapshoot. But a better quality part will, whatever its character, offer more of some basics, whichever particular subset they happen to be.

That is funny. I dislike Miflex. Far too forward sounding to me. I replaced them in my integrated amp and past tube dac with Odams. The Odams were clearly better in every sonic parameter.  Now Vcaps do take awhile to burn in. Longer than Miflex. Wonder if he did not give them 150-200 hours! If not, that is why. The Odams can sound constipated and bass shy initially. 

I know the Miflex very well. Just can’t get past how pushed forward the stage is. I know two other builders who now offer Odams over Miflex based on listening tests.  
Go figure. 

There is no such thing as universally better parts—the right part is a matter of both particular design and taste of the listener.  That means trial and error and a willingness to accept that the majority of changes will be for the worse (the original builder having already chosen something that works reasonably well, your somewhat random change is likely to be positive).  It is not necessarily the case that parts that are popular will be to your liking.  
A custom builder I know passionately hates Mundorf caps and can tell when they are in amps and speakers just by there particular sonic signature.  Another builder I know was in some distress when the “cheap” Black Beauty volume pot was no longer made because they worked so well in his designs; cost was no object in his builds, yet he did not like some very expensive alternatives. 

Sometimes the right parts are very specific to the way the product is used.  A local dealer/builder upgraded one of his customer’s amps with Blackgate coupling caps. The customer had auditioned and liked the upgrade in the shop, but when he took his amps home the result was disappointing.  It took some effort for the dealer to determine that the owner tended to listen in short sessions while the dealer’s gear is on for many hours before the customer heard it.  It turned out the Blackgates needed the long warmup to sound its best in this particular amp.