That's all good.
A Marshall cranked with an SM57 in front of it has been the method of choice for many a professional engineer . . . and is probably what you're hearing on a lot of our favorite records.
Before I sank any more money in to another mic, I'd make sure to exhaust every other method; switch pickups, lower the gain somewhat (always use less than you think you need), elevate the amp to ear-level (or crouch down so your ear is at amp level) . . . and while listening that way, try messing with the EQ and gain knobs until it sounds right to you.
If you're absolutely certain it's a mic thing, then in the under $400 category, I'd be looking at the following:
* Electrovoice RE-20, used.
* Shure SM-7 (quite a step up from a 57, from my experience)
* Beyer M88
Maybe try a ribbon like a Beyer M160.
I'm not sure that I'd recommend a condenser for heavy guitar, but a few you might consider:
* Audio Technica 4055 (used).
* Blue Baby Bottle
* Groove Tubes MD-1B