Warren Haynes option is a bright cut switch on the Overdrive Channel and only works at pregain settings less than 4. When you lower the pregain setting, the bright cut is more apparent.
I've seen this repeated over and over on numerous forums and I just don't get it. I picked up a 93 SLO100 recently that didn't sound the way I expected. Once I figured out that the bright cap on the overdrive channel had been removed, and replaced it, BAM! there it was. All the SLO goodness I was expecting.
The poster above did say that the bright cap is less apparent at lower volumes, but I can assure you, at least with my amp, that the bright cap has a significant effect on the tone at just about any gain setting. The amp sounds completely neutered without that cap.
I also removed a depth mod while I was at it, and the amp now sounds absolutely stellar.
I guess I'm in the camp that thinks the SLO100 sounds best stock.
One other thing I discovered was not to use New Sensor 12ax7's with the spiral filaments in V3 & V4 of the SLO, which are both cathode followers. When I got the amp, it had all Tung Sol's in the preamp, and it hummed like crazy. Figured since it was a 93, it needed filter caps, but swapping out the Tung Sols fixed the issue.