Your turntable is nice, it was the TOTL Yamaha belt drive.
Here is some info at the VE library, unfortunately no users manual or service manual but it has a linto to an informative brochure. http://www.vinylengine.com/library/yamaha/yp-b4.shtml
Before getting into mods I would do the basic maintenance. I would replace the belt, clean out the bearing and add fresh oil. I would give the motor a drop or two of oil. Make sue the motor spins at the right speed and is quiet. You can download a strobe tool for this. I would check the feet and make sure they are in good shape and doing their job of vibration isolation. Make sure the TT is level and mounted away from external vibration as much as possible.
If you decide on a new cartridge there are plenty of good options. Shure M97xE, AT 95E and ortofon OM 10 are all in the $60 range. At ~$100+ there are many more that would be compatible with your tone arm.
If you receiver has a low quality phono section (and it probably does if it is a home theater receiver) then adding a phono stage (i.e. phono preamp) would really make an improvement. I would rather do that than a bunch of hokey add ons to the TT.
Speakers are another place to maximize bang for buck. They are critical for a good sounding system. Nice thing about your receiver is you can get very good used bookshelves and a small subwoofer and really get some good sound.
Take your time and listen as you go. Get the basic TT working properly first before sinking $$$ into other mods and things. I think your deck can be as good as a $500-600 new one if you get it set up properly with a well matched and properly aligned cartridge.
good luck, Bob