There are several good mod guys out there, as I'm sure others will point out, but the one I'm most familiar with is Ric Schultz at Electronic Visionary Systems(www.tweakaudio.com). If you read the info on his site you'll see a pretty convincing argument why, all else being equal, a one-box player is superior to using an outboard DAC(in fact he recently stopped producing his incredible Millennium DAC, which I use, to focus on modding DVD/SACD players). Ric also gives a nice overview of the different level of mods he does. One thing Ric does that I'm not sure many others do is to take all op amps out of the output stage and replace them with a discrete mosfet setup. Suffice it to say he does a lot more than simply swap individual parts, but you might also incur a bit of a wait to get your player modified--likely worth the wait though. Best of luck.
Tim