Hi Murph,
First, if your Denon has "phono inputs," then it already has a built-in phono preamp. No outboard phono preamp will be required.
I doubt you have access to a stylus inspection microscope--formerly, elaborate audio retail stores had these on site--so it's impossible for you to determine how worn the stylus is on the Panasonic phono cartridge. Since the latter were mediocre, I'd suggest replacing the cartridge with something from Shure or Grado. Both US brands still make the most linear and high-quality cartridges at affordable prices. Check out the Shure M97HE or a Grado model under $100. Stanton is also an excellent US-made cartridge brand. The Shure cartridges have the advantage of a damped stylus brush that rides on the vinyl ahead of the stylus and reduces the effects of tonearm/record-warp interactions.
Regards,
Alan
