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

Yes Solen Fast caps are made in France. They could very well be used by QS. They are the very definition of modest quality…at best. Dull and flat sounding compared to the better ones mentioned here. Used by many builders because they are very low cost. Some builders do have real cost constraints to contend with. 

I agree with granny again.  Miflex are really great caps but they are forward (not harsh though) in their presentation.  I've never heard an ODAM sound congested.  At the risk of sounding VCap biased, maybe the Odam was revealing something in the source component?  

I also love the British made ClarityCap.  I like them (personally) a touch more than Audyn's higher end stuff.  The good to best ClarityCaps can sound holographic and surreal while being pretty neutral.  

Auricaps are great on a budget. 

Mundorfs are overall great caps. I just like the other options for my taste. 

If you really want to experiment, you could buy the Musical Paradise preamp, which allows you to roll coupling caps as the unit uses binding posts (like speaker posts) for coupling caps!  Very cool.  I picked one up for $500 and use I to test out cap sounds.  I know, though, that the results are a skewed by the circuit too, yet this preamp has a fairly simple design. 

For clarity and to conclude my posting, my post is to offer alternative viewpoints in case the OP wanted to try something different. Considering the same issue is not there with other caps, I would rule out other variables. No intention of bumping elbows here.

@grannyring Yes Solen Fast caps are made in France. They could very well be used by QS. They are the very definition of modest quality…at best. Dull and flat sounding compared to the better ones mentioned here. Used by many builders because they are very low cost. Some builders do have real cost constraints to contend with.

Yes, "modest quality" at best. A little "dull and flat" is a good way of describing it. The bottom line is the stock caps in the amps are built to a low price point. Well built amps and super affordable, yes. 95% of the regular customers are okay with the good sound and reliability in stock form. Makes good business sense for QS. The retail price is far below many others, and this is partly why.

The other 5% of us over the past two decades+ who’ve upgraded caps want to get even more out of the venerable QS monos. Upgrading with high-end well made caps (many mentioned on this thread) and better input/driver tubes allowed me to keep the amps, liking them now much more than originally anticipated. Amazing what these small and affordable changes did for the sound and added musical enjoyment. For added confidence, read through old and recent threads on AudioAsylum, DIYAudio, and other forums and you’ll find members who’ve completed these exact same upgrades on their QS amps with success. If you are going to do it, buy the best caps you can, it will likely pay off for you. Best of Luck.

excellent discussion and exchange of ideas from several experienced, well meaning, classy, knowledgeable folks

a pleasure to read and learn from