I Just had my Wadia 301 upgraded by Reference Audio Mods and the imrprovement is dramatic. Great bang for the buck IMHO to upgrade through mods rather than buy/sell.
For about $650, I got the Superclock 2, a Jensen cap, and about 50 Black Gates installed. It's still breaking-in (BG's are notoriously slow), but it sounded better than stock upon first use, and it's still getting better (I'm only about half way through the 300 hour break-in period).
The improvements are many- more weight to the presentation, less digital sounding, better soundstage and timbre. Pianos now sound rich and full, with proper weight, whereas before they had a slightly glassy sound that was annoying.
Reference Audio Mods really represents a value in upgrading, IMHO, since they also sell the parts. In fact, I didn't even know about them until I was searching for the best prices on Black Gates for my amp, and another A'goner (Fiddler) recommended I check them out. Not only did they have the best prices, they also installed them for the same price. I had my amp, then my CDP modded and both were big improvements. Even in components costing twice as much as mine, you won't find BG's, Jensens and Audio Consulting silver hook-up wire inside. If you want to enjoy the benefits of that level of parts, you'll have to put them in yourself.
I never modded anything before doing my amp, but now I'm a believer. If you've got something you like, you can make it better for much less than buying a new component. And just to disclaim this, I have no connection with these guys other than as a satisfied customer who felt he got good value and great sound for the money.