I'm using an 80s-vintage Harmon Kardon T-45 turntable, which now has a
same-vintage Grado M+ cartridge. The amp is an NAD 713 receiver, the
speakers PSB Image b-25's, with a PSB subwoofer next on my list of
additions.

Are there cartridges in the under $150 category that will significantly
upgrade the sound, or should I just seek out a replacement stylus for
the M+? The sound now is quite pleasant but a bit muffled seeming, so
more detail and dynamic range would be desirable, so long as the overall
"musical" quality isn't compromised. I listen to many kinds of
music--chamber, blues, opera, funk, pop, arty rock. I also can detect a
bit of hum with the turntable--I don't know whether the cart has
anything to do with that.