Well.....Here are the mods they list for the 5150II head and I'll take a guess at each one;
Bias Mod-Since it already had an adjustable bias, they extend the range (I had this one)...it does make for a warmer sound, but the tubes will not last as long.
Restore Lead Channel-I have no idea....VB??????
Tone Shift Mod-They might be playing with the Q points of the bass/mid/treble/presence knobs...I like them stock, but I can see where some people might want to change them.
3 Channel Mod-Well it basically has 3 channels (clean crunch, and lead) so I'm not sure
IEC Plug Jack-In case you don't want to use the standard 1/4 inch guitar cable
Noise Reduction Circuit-I'd ask a lot of questions about this one....might be nothing more than an integrated noise gate
. You need to keep in mind that these amps have so much friggin gain they are noisy at high volumes period. I used a noise gate, so does Ed, and so should you. Load boxes, attenuators, etc do nothing for this amp...because it has so much friggin preamp gain at any volume. They put a gate in the JSX at the factory.
Half Power Switches-this would be cool, but you can do the same thing just by removing 2 of the power tubes. I have done it many times....the amp is still louder than HELL at half power, but has a lot less low end.
Bottom line-Mine sounded awesome at any volume, but I had loads of external gear hooked up to it to get my sound. They guy I sold it to loves it!