What PC do you use...

with your PP500/300?
Of course PC performance is system dependent. Just went through extensive trials (8 cords)over 4 months with the Premier. The best cord was the inexpensive VH Audio Flavor 4 (Furutech connectors). Some well regarded cords that were significantly more expensive were part of the trials.
Cerious Technologies with my Cullen modified P500. Bested everything else by a substantial margin including Electraglide Ultra Khan II and PS Audio Plus.
Fusion Enchanter on my PS300!

Can you describe the sound before and after the mod?
Honestly, its about 11 months since I had the mod done, so I can't recall specifically what the mod did sonically, except to say I was extremely happy with the decision to upgrade at the time. In addition to better sound, the mod added a remote and made the unit run more efficiently. The fan runs at a slower rate so it is a bit quieter. I can also tell you that when I spoke to Barrows at PS Audio he told me that the modded P500 was competitive with the newer Premier. Since I use the P500 for a speed contoler for my VPI Aries II with the dual motor flywheel, it also takes the place of an SDS. I personally thought the upgrade a wothwhile investment.