Lack of vinyl bass and midrange

With a great rig like yours, I think you really owe it to yourself to buy a modern cartridge. Without naming specific models, I'd recommend:

Transfiguration
Lyra
van den Hul
ZYX
Shelter

and if you can get the Monster rebuilt for two to three hundred dollars, great! Keep it as a spare. The Stylast solution is a little like using Armorall on old plastic, i.e. you cannot re-polymerize old dried-out rubber/plastic products. You can only temporarily "knit up" the unravelling molecular chains, but they will never give you the original performance, and will come undone again in relatively short time.

1...Verify that pickup wiring is not out of phase between channels.
2...Compare left and right channels (using a mono LP if you have one). If they are the same suspect the pickup. If one channel is bad the RIAA equalization in the preamp might be the problem. (Unlikely that the two circuits could fail the same way). Beg, borrow or steal a different preamp to try out.
3...Play your LP on someone elses system so you know how itsn bass and midrange should sound. Maybe you are just hearing a LP/CD difference.

I've found that you CAN bring some carts back from the dead. Had some Sonus stylus assemblies in a junk drawer for at least 25 years and tried them on a Sonus body I found on bay. Dead like mackeral. Figured I had nothing to lose by slathering some "Re-Grip," usually used on the rubber parts of a reel-to-reel deck, on the rubber suspensions of the styli. After playing half a dozen LPs, the Sonus really started to sing. Now it has pride-of-place on my Lenco/SME IIIs setup. Good luck, Dave

Ditto Dave's experience: I treated my Kiseki Blue to soem Stylast some 4 years ago when it had the same problem (had been tucked away for a few years) and it's still going strong! The flip side is: Van den Hul anyway refuses to work on cartridges that have been so treated, so if you are thinking of sending it in for a rebuild you might be better off sending it right away before treatment. Again if it is destined for the trash bin anyway then there's nothing to lose.