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

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.

Call Mike at QS. He has a very good ear and can help your quest. Although he might say don't change anything. 

When I rebuilt my Dyna 70s into VTA 70s, I added the VTA board, new power supply caps, fresh tubes throughout, and premium caps in the audio stage.  The biggest changes since then have been from rolling tubes.   I settled on RCA clear top 12AU7 driver tubes, 12BH7 in the center and Gold Lion KT66 output tubes. 

Some vibration isolation can't hurt.

Some vibration isolation can't hurt.

Put springs under the amp. Plenty of threads here on the subject.