Which amp from ceriatone has the highest gain?

Well, high gain, low gain, etc. usually describes how much gain and saturation is available in the preamp.  Nik doesn't yet over anything with a lot of cascading gain stages, like an SLO or Uberschall.

I would guess the Overtone Special edges out the JCM 800 and TW Expression, but the OTS is all about smooth, saturated preamp gain for blues, jazz, classic rock, etc.  It's not a high-gain machine like an Uberschall or anything.  The JCM and TW are more about overall fat rock crunch when cranked to include power tube overdrive.  If I had to choose from this list for metal/hard rock, I would lean toward the JCM or TW.

Schematic Heaven has the SLO schematic and some Mesa ones.  I think both are possible with the 2x12AX7 available to the preamp of a JCM.  It would really be a complete different preamp circuit , instead of modding the Marshall one.

Since it would only be a one channel amp with a MV (the SLO uses two more 12AX7s for a separate clean channel), the JCM would work better than a Plexi.

Check this out, it's a 4 gain stage caper without adding an extra tube. Looks pretty easy to do. I haven't done it yet to my plexi 100 Ceriatone, I'm still procrastinating, I have other mods to consider as well but have a listen to this clip from Rockstah.
http://www.soundclick.com/bands/page_songInfo.cfm?bandID=503988&songID=3877112
If the mod sounds even close to this, I'll be rapt but you just have to take a punt n do it. Someone please do it and let us all know how it sounds?

Hmmm....would not the new 2550 be a high gain offering? Not Uberschall territory but more gain than the 2204 or jtm or 1987?