Is upgrading pre worth it?

A huge difference in my opinion the preamp makes a bigger  impact on sound quality than an amplifier. And going from a receiver to even a modest well designed tube preamp should really be a quantum leap forward in sound quality.

Its not close,  if the funds are not a problem,  I wouldn't blink to getting a good pre

You're not hearing what your amp is capable of. I agree with others that the preamp has more to do with the sound of a system over an amp; they're both important. I've heard ok amps sound very good with a great preamp, but not a great amp sound great with an ok preamp. Try the Rogue pre... can't go wrong with that brand.