Lack of vinyl bass and midrange

Ditto the suspension. You could try dribbling a drop of Stylast in the inner workings of the cartridge to loosen the suspension, it's worked for me before with no problems. Or try a drop of other rubber revitalizer which others have used successfully. This is if the cartridge is toast and destined for the rubbish heap anyway, if you want to keep it in the long haul then safest would be to send the cartridge somewhere for rebuilding.

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.