RAM (reference audio mods) and Kern (audiomod)have very good reputations. I have also heard good things about (VSE) vacum state electronics. I believe music technologies (Bill Thalmann of cj fame) is the north american installer of their mods. Sonically, depending on the mods chosen, you should hear a substantial improvement in the players performance. But resale value will most likely take a hit. And obviously you will void the remaining portion of your warranty. I think Kern advertises that he backs his work with a one year warranty.
Esoteric Mods
I own an Esoteric P70/D70 Transport and DAC, I am considering having both modded. I have been doing some limited research, however I would like to hear from anyone who has had their Esoteric units modded to get some feedback. My units are still under warranty for another 6 months. I would like to know the names and websites of some reputable companies I can contact. Bottom line is it worth investing into. Thanks for your help