Mitchell MM100 problems

I recently purchased a Mitchell MM100 electric guitar from guitar center. It is a barebones short-scale version of their M series guitars. I did have to replace the input jack as the insides were getting loose, and since the jack required is of the type where u cant really get at the nodes that touch the cable end, I couldn't just bend them a touch to make it tighter. Anyway, once I got it replaced, it seems to have an extremely nasty hum now. I thought it was a ground issue, but upon testing the grounds continuity everywhere, I am at a bit of a standstill. Any advice on what to try next would be appreciated. I have a nicer set of Schecter pickups I was going to try next, but just thought id poke around here first to see if anyone had any better ideas. Tyia.