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

In my opinion the type of upgrading you are proposing makes sense for someone who is interested in doing the work themselves (DIY) as a learning experience along with hopefully improving the sound knowing that these changes may or may not improve anything and may even degrade the sound causing you to roll back some or all of what you replaced. The big problem with having someone else do these changes is if a number of items change at one time and the sound does not improve and possibly degrades then, which upgrade created the problem? You need to upgrade one item at a time and listen for a period of time to determine what the differences are and if they were worth keeping. Then do the next. This can get expensive if you are paying someone to do this work, which makes me say it is not worth the time and money. If however you are doing this because of a perceived flaw in your amplifier that you are hoping to improve on then I suggest selling the piece and using that money along with the money for parts and labor that you were going to spend and use all that money for another amplifier that sounds better. On the other hand if you love the amplifier then just work on upgrading the peripherals such; tube dampeners, NOS tubes, higher quality feet, replacing power / interconnect / speaker cables with higher quality ones that sound better to you and possibly upgrading your electrical receptacle along with installing a dedicated circuit to power your amplifier. I believe this route has a better chance of seeing noticeable improvements and would cost about the same, depending on how wild you chose to go. The quicksilver line of amplifiers are well thought out and perform very nicely so paying someone to mess with them does not seem to make sense to me.