I had the "Rod Mod" done on my RI Bassman in end of August by John Kinder in Moreno Valley, CA (909)242-5923. Yeow, it's a beautiful thing.
The mod made a huge reduction in feedback allowing the amp to be cranked louder while standing directly in front. The tone is rich and not so much dirty as singing, very compressed, like Piazza. As soon as I spoke thru the mic I thought, oh man, that sounds just like when Piazza sings or whoops thru his harp mic.
Basically he makes the amp sound bad for guitar by eliminating sustain which then allows it to be played loudly with a microphone without feeding back. So the sound is dry, in that, when you cut off a note, it really stops fast. But now I can add much more delay (with a pedal) than previously with less feed back and get as "wet" a sound as desired.
Also, he told me I might have to blow harder with this mod. I didn't understand that until I played it. It seems I have more dynamic range for a given volume setting on the amp. I can play softer with the amp cranked without backing off on the mic volume.
This this is a great mod. I did it buy mail, as I live near New York. I learned about it thru this list, no connection to Kinder. Costs $180+shipping, ouch. I asked him to just tell me what he changes and said I can easily do it myself. But he does not want to give up what he's learned thru lots of experience. I can respect that.
So don't go after those guitar amps with great sustain, believe me.