"Change" gain and mid range .. what do you mean by that?
You really need to give more info for us to be able to be helpful.
Adding gain can be done with a higher gain tube, or with a more "percieved" high gain tube like an EHX.
Likewise, less gain can be gotten by sticking a cleaner tube in there or even an 12AT7.
Mid range ... this depends on what you want.
I'd start by sticking a fresh set of tubes in there plus in the V1 of the amp.
On older modules this might bring some definition back that you're missing.
But then I don't know wether that is the problem.