What should I expect from ....

a phono preamp upgrade? I am currently running my analog (MMF-5, Denon DL110) through the stock preamp in the Musical Fidelity X150. Mainly real Jazz. My question is- what type of improvement should I expect from a stand-alone pre? My budget is tight. Anything for 500 or less that is worth considering?
At your price piont, you might be better off getting a better preamp with built in phono stage--maybe something like an audible illusions M3 or M3A (w/ mm phono stage). Once upon a time, I added a well-regarded 500$ phono stage (monolithic) to a cheap integrated (cambridge a3i). The results were slightly better bass and a cleaner sound, but not really worth 500. Later, I used the phono stage with a conrad johnson pv10al. On a lark, I bought a used audible illusions 2D preamp with built in phono stage. To my ears, it sounded significantly better than the more than twice as expensive monolithic/cj combo.
My guess is you wouldn't really benefit that much unless your willng to upgrade other componenets. It can be a viscous cycle and very costly and end up never being satisfied. I guess what i am saying is if your pretty happy with what you have let it go and enjoy your current set up....Hope that makes sense..