I'm not quite sure to what extent it has been tweaked. I know that Mike has re-voiced the gain on both, the JCM side, as well as the Beast side. He also added a toggle on the back that shifts the frequencies, I believe mainly the mids from low mids through high mids. Think thats a 3-way toggle. He also added a resonance knob on the back as well that allows one to control the bass response. Mike made the built in noise gate to work on both modes of the amp instead of just in the Beast mode.
I originally asked Mike to do the Hulk mod to my JCM 2203. He told that it is not really a mod, but a complete re-build. He stated that it was very time consuming, expensive, and after he made the NATAS he discontinued the Hulk Mod. I believe the Hulk Mod was the precursor to the NATAS. I had told him that I really wanted the SATAN head, but it would be out of my budget range for awhile. Thats when he referred me to Ralph of Mod Squad, and told me about the Fortin 2203KK. He stated that it was really close to the aggression of the NATAS.
At first, I wasn't into getting the modded 2203KK, but after discussing it with Ralph, I became more and more interested. I had already thought to try the stock 2203KK because I loved the JCM 800, but wanted a tighter, more aggressive amp. So after a trillion emails, and some phone calls back and forth from Ralph, I committed to it. Now its in the mail and will be here next week. Ralph really hooked me up, and he is a really bad ass guy to deal with.