Another way to upgrade is to spend your money on the very best cartridge you can afford. I am not that familiar with the Rega line myself, but I am sure that the analog gurus on this site can help you pick out a really nice cartridge for your P3, one that will maximize its strengths. You should figure out what you are willing to spend on a cartridge (And then double it! Sorry, but they do tend to be expensive.)
Good Luck!
Good Luck!