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

grannyring has good advice above.  I like the VCap Odams much.  They are also easy to work with size wise. But you'll need to tell your tech that their bodies are somewhat conductive. I wrap them in silicone tape prior to install.    I would simply start with the coupling caps and output caps (typically located right before the lines running to RCA outputs).   If it's a preamp you are upgrading, you can match your output cap better to your amp if they are of different brands.  There's a nice calculator on the VCap website for this purpose. 

I was never able to find a tech that wanted to assist with modifications.  It's kind of like taking your car to a service place and asking them to install specialty muffler, brakes, etc.  They typically aren't jazzed to do it and they look at the vehicle as if it were an appliance--not to be touched from the engineer's design. 

I've experienced far more meaningful results upgrading caps/resistors than I have playing with hookup cables/power cords (I do that too but I just hear more of an improvement from the other stuff).  

One warning you didn't get yet---It's slightly addictive.  I spent $900 upgrading my crossovers on my loudspeakers.  I bought Cornwall IVs but would have loved Volti speakers (didn't want to spend $10k).  The crossover upgrade got me where I wanted to be.  

Modifications can keep me from upgrading by allowing me to get more out of a design I already love.  A Ford Mustang or even Chevrolet Corvette are built to a price point--they are not a Rolls Royce. So there's a good foundation to make improvements to your specialized taste.  Enjoy your journey!

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.