I know this might sound very basic but it kind of sounds like you have a gain issue. Quite often, people will make a mistake and buy a moving coil phono preamp and mate it with a high output moving coil cart. The result is too much gain and will have a muddy distorted sound. Even though the cart is a mc, it should go with a moving magnet phono pre instead because of the high cart output. I'm not saying that is definitely your problem, but just something to consider.
Something else to consider is that all Rowland gear that I know if is balanced. I believe most, if not all, Rega is not balanced. You may have to do something to the Rowland to accept a SE input. (button, dip switch, pin change, etc). Given that your system seemed to be working for a while, you may have accidentally tripped a switch you did not know was there.
The only other thing I can think of is cart. alignment and/or setup. If you have a Rega arm and Rega cart, it should have 3 screws, as opposed to the standard 2. The 3rd screw aligns the cart to Regas own, proprietary alignment specs. Mabye the screw is missing or you have a different brand of cart. If that is the case, you will have to align everything by hand.
That's all I can think of. I didn't want to repeat what was already stated in the other posts. Good advice, as well. I haven't been there in a long time but I do know In Living Stereo pretty well. They are very reputable. Its hard to trust people in NY, but I do trust them. I do not know if he still works there, but if you can, ask for Jonathan. He knows vinyl very well and I can't imagine that he, or anyone else who works there for that matter, can't fix this for you.