Question About Capacitor Upgrade in Tube Amp

Lots of great recommendations and suggestions on upgrading.  I for one really believe in it.  I went from Janzens to Duelund copper CAST coupling capacitors in my tube-preamp and at first, I thought not such a good upgrade, but after ~100-hours of break-in, wow...  Increased inner detail and for lack of a better term, body in the music. My good friend has a pair of Welborne 300-B SET's that we're going to do a component upgrade to sometime next year.  They sound surprisingly great for their cost and what they are.  Simple circuit and all...  I believe they will sound notably better with these upgrades.  As many have said, please don't judge the switch until you've given some time for break-in !  

There was a very very slight smoothness added in the very upper treble after about 20 hours run in versus very sharp and defined treble notes when caps were first installed. 

Just last night I installed NOS RCA rectifiers and preamp tubes. So it will not be possible to report back any more cap break in changes. The resolution of the music increased along with the smoothness and musicality with the NOS tubes. I also tried new production Tung-Sol but they sounded gritty in comparison. Maybe new tubes need to break in as well so I am running them in another amp for now.

I am waiting on a delivery of PSVANE 300B TII matched pair power tubes as the last upgrade.

This is THE amp for my Klipsch Forte IV speakers. A perfect match.

Would I say it is the best amp I have in every case - no. The KT150 amp is better with my Dynaudio speakers for example. So system matching is still more important than these other tweaks.

Good for you if the Mundorfs work for you long term. Every time I've tried Mundorfs, initial impressions good, over time always heard spot lit high freq, rather artificial sound quality result over longer term. Hopefully these work for you long term, caps exist for all needs.

Capacitors have their own technology, dynamics and very often audible characyeristics. Anybody who disagree most likely never replaced one in the audio path of their high-quality equipment piece.

As for capacitor used to shunt DC power supply, they are necessary to eliminate high-frequency noise present there, but never affect the sound quality as much as a different coupling (serial) capacitor used in the audio path.

I'm also a strong believer in using high quality film caps. I've had good luck with V-cap ODAMs (very neutral and clear) and Miflex KPCU (a bit warmer with a beautiful midrange tone). 

A couple things to keep in mind. These caps all take a few hundred hours of break-in to sound their best. And some will actually sound worse during the break-in period. 

And most important, these amps have LETHAL voltages inside. Capacitors can hold a lethal charge for a long time. Make sure you are thoroughly familiar with the safety procedures before playing around inside a tube amp. The OP mentioned doing DIY guitar amps so is probably already familiar, but anyone else contemplating doing this kind of upgrade should use extreme caution.