Cartridge Question

I'm new to all of this, so pardon me if I ask a stupid question. I recently changed the cartridge on my Music Hall MMF-7 from the stock 5mV Goldring Eroica (MC) to an 8mV Ortofon 2M Black (MM). The reason for this was because I was told by the manufacturer that my new Rogue Audio Cronus Magnum needed a higher output cartridge for the internal phono gain stage. While the clarity of the new cartridge has improved dramatically, the warmth of the tube amp basically went away. I decided to put on several different genres of vinyl in an effort to figure out what was ruining the entire reason I purchased the tube amp in the first place. As best I can tell, there is some distortion occurring at higher frequencies; thus what I suspect is drowning out the lower warmth I enjoyed so much when I first received my amp. Any suggestions?