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.

@axo0oxa Thanks for your caution. It creates some indeterminacy to do multiple caps at once, but if it works, it works big. That's a gamble, and if there weren't other quicksilver owners testifying to the benefits, it would be perhaps too much. But there are and so it's not. I'll let folks know. My mind is made up. 

Good for you!  Only personal experience will allow you to determine whether benefit worth the cost. I expect you'll find the benefit well worth the cost using any of the top flight caps. And aren't all moves in audio taking a chance, purchasing new equipment is as much, perhaps a greater risk. Too many allow fear of failure to interfere with progress.

No on upgrading transformers. They are good, high in quality and function.  

Good to know. Thanks. @decooney 

@hilde45 Did you ever complete your QS cap upgrades? Reading this thread its not clear which QS model you are working with. If you have a newer QS item I do believe the capacitors if white in color are already Film and Foils if you replace these with any kind or metalized polypropylene cap you're most likely doing a down grade. In almost all cases a film and foil cap will be sonicly superior to a MP regardless. Mike at QS now has available on the QS website Film & Foil capacitors to replace common QS sizes .27uf, 2.0uf, and .056uf I believe. If yours are yellow in color or not identical to the new QS caps you should consider the QS labled film and foils. Im currently updated an older QS full function preamp that seems to be using older QS Metalized Polypropylene caps. I will be using the newer white film and foils from QS. You can view my progress on lowpower audio on youtube.