First, congrats on realizing AVR's and mp3's are great for sound effects and convenience, but for pure music reproduction, dedicated stereo systems are the best way to go. So, nice system you have put together to start enjoy music the way it's meant to be heard.
WRT the next upgrade, the transducers (cartridge at the front end and speakers at the back end) in a system will always give the biggest impact on how a system sounds. So you have probably answered your own question in your initial post - upgrading the cartridge from a 2M Red to a 2M Blue should provide the best bang for your buck that's within your current budget. A nude elliptical is a big step up in performance from a bonded. Next would be the speakers and that would strengthen the two so very
important transducers in the chain.
Good luck.
WRT the next upgrade, the transducers (cartridge at the front end and speakers at the back end) in a system will always give the biggest impact on how a system sounds. So you have probably answered your own question in your initial post - upgrading the cartridge from a 2M Red to a 2M Blue should provide the best bang for your buck that's within your current budget. A nude elliptical is a big step up in performance from a bonded. Next would be the speakers and that would strengthen the two so very
important transducers in the chain.
Good luck.