I owned a Martin Golub modded JMP 100watter, and the amp was cool but i ended up selling it off eventually because it wasn't 100% what i wanted tone-wise. It had very little taper on the Gain pot, meaning that the gain levels didn't really change much regardless of where the Gain pot was set. The mod doesnt include a Depth knob by the way.
When i sent the amp in to LASD, i was told it would be like 6-8 weeks roughly because they had a small backlog. I didn't get my amp back until 4 months later, which wasn't fair IMO.. considering the turnaround time i was told in the first place. The tubes were also smashed on arrival, so i had to replace them myself..
I now have a JCM800 50watter, which is completely modded. The mod was done by myself, and Mike Fortin and I spent a lot of time working on the design. He's a very busy guy, but he spent every chance he had, helping me with the mod design.
Mike offers this mod for Marshalls (mention my Jose800 amp to him) and i can honestly say that the mod sounds every bit as good as the Golub mod i had if not better. For my tastes, i prefer it to the Golub... hence the reason for me having it now and letting the Golub go. As Ralph posted, he feels the same way too!
And just to set the record straight, I'm not recommending you go with Mike Fortin just because i got a rough deal on the situation with my Golub mod. But it's because i honestly believe the Fortin stuff is the best stuff around. I have had tons of killer gear come and go over the years, but this modded 50watter will NEVER be going anywhere. End of.
It excels at the Demartini and Aldrich type sounds, and that's the kinda sounds i go for anyway

There's a short clip in my sig, demonstrating the tone of the mod. It sounds better in the room though! I'll be making more clips very soon!