I've heard mods done to a variety of DACs, amps and preamps. John Hillig and John Wright are masters at this and their mods are absolutely worth the very reasonable sums they charge. However, most of their mods are in the sub $1,000 range.
I think the challenge with multi-thousand dollar mods is judging the return on one's investment. If one is going to keep the gear, then it's strictly a subjective call; if you like the sound, it was worth it. If, however, you plan on re-selling the gear, I don't find the super expensive mods to be a "sound" investment because you rarely recoup the investment when re-selling.
To get a taste of what a moderate modification can do, replace your stock fuses with some Hifi Tuning or Synergistic fuses. Be careful, though. You might get bitten by the upgrade bug. ;-)
I think the challenge with multi-thousand dollar mods is judging the return on one's investment. If one is going to keep the gear, then it's strictly a subjective call; if you like the sound, it was worth it. If, however, you plan on re-selling the gear, I don't find the super expensive mods to be a "sound" investment because you rarely recoup the investment when re-selling.
To get a taste of what a moderate modification can do, replace your stock fuses with some Hifi Tuning or Synergistic fuses. Be careful, though. You might get bitten by the upgrade bug. ;-)