A lot of great advice here. I agree with some - MB cabs are 'woody' and uninspiring to my ears, but then I've always played active basses with soapbars through them and been underwhelmed (seem to remember playing a jazz with DiMarzios and being disappointed too?), so I may be delighted by the results with an old P.
If you want real sparkle, change the cabs, but a string change/outboard preamp/pup change could make all the difference you're after.
Cheers,
Tobie