Lack of vinyl bass and midrange

Thanks for the responses. I'll try the Stylast. Where would I get it rebuilt?

I think the Stylast will work, dribble only a very small drop into the inner workings, let it sit for a few minutes, then re-mount it and let it play a few records. Both Van den hul and Benz offer rebuilding services, as does Len the Cartridge Man (of Music Maker fame), I believe Benz is the most reasonably priced of these. Lowest price of all with good reports is Expert Stylus Co. in England, but apparently hard to reach, check out Vinyl Engine for info on all of the above.

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