I was going to offer the same explanation that @mijostyn did - the extremely high output of the Rega cartridge might be overloading your phono stage. If you have lower gain settings on your phono pre, you might try that. Or if you have another cartridge with lower gain, you could install that to do a comparison. If you don't have another cart, you can pick up something cheap like the Audio Technica AT-VM95C for $39 to give it a try. And having a cheap, backup cart is never a bad idea in case you accidentally damage the Rega cart.
Possible cause of harsh mids?
Once in a while you get that used album that sounds good except for a static like distortion on some more dynamic vocals, horns, guitar. Just typical for some old albums? Listening to Corea - The Leprechaun. It's a very good sounding album, well balanced with great dynamics. But, that annoying distortion rears its ugly head once in a while.