I'd like to suggest JCM 800 2205 or 4210 (smaller chassis than the head version and no presence control) based on the last version of the circuit (model year 1988-1990). In the last version Marshall resolved the issues with channel bleed by changing the switching mechanism and also they are known for having a bit more gain on the boosted channel.

I think this would be a excellent compliment to your existing offerings of JCM 800 style clones. There are some that turn their nose up at anything other than the original 2203/2204 's however the later model 2205/2210's are also well regarded many especially those looking for a bit more versatility than the single channel models.

If you chose the smaller chassis model (4210) I imagine you could trade out the Reverb control for a Presence control. And of course having external bias test points and trim pot(s) would be a huge improvement over any stock JCM 800.