I agree with Swampwalker and Phasecorrect - your system looks OK and you would have to spend a bit to make a large improvement. But you should check to see if everthng is setup correctly. First, check your cables. Then, check that you are using the correct cartridge setting on the back of your Phono Box II - it should be set to MM. Then check your TT setup (cartridge, turntable isolation or coupling, etc).
My rule is (and some might disagee with me) that all else being equal (meaning no setup or mechanical issues), your biggest improvements in sound will come from the "outside" in. For analog, that means that biggest changes in sound quality will come from improving cartridge/arm first (they must be considered together), then TT deck, then the phono stage, then preamp, then power amp; with diminishing improvements in overall sound quality as you go down the list. As with all rules there are exceptions, and YMMV.
My rule is (and some might disagee with me) that all else being equal (meaning no setup or mechanical issues), your biggest improvements in sound will come from the "outside" in. For analog, that means that biggest changes in sound quality will come from improving cartridge/arm first (they must be considered together), then TT deck, then the phono stage, then preamp, then power amp; with diminishing improvements in overall sound quality as you go down the list. As with all rules there are exceptions, and YMMV.