Here's how, SPL from mains went down from volume pre-set, which required 8db volume control gain to recover.
The only way you could loose 8db, is if the magnets in the cartridge became partially demagnetized and this can and does happen. I bet though you have lost more than just output though, and have a drooping top end as well.
If what you say is correct, then that cartridge is NOT working to spec. most likely.
Causes could be stray AC flux from the turntable motor if it is AC drive or some type of motor driven from a servo controlled generator that produces square or saw tooth waves. It can be from getting the turntable too close to a speaker. There are a lot of possibilities as to how this could happen. It may be the magnets were made of poor material and have lost magnetic flux over time.
Of one thing I can be certain loss of output of 8 db, which is not subtle actually but a major problem, and the getting smoother suggests a roll off of HF, is NOT due to cartridge break in.
I have not used one of those Jaco replacements and got my stash of genuine Shure parts before they became hard to get. I bet that replacement stylus assembly which contains the magnets is now out of spec and faulty. If what you say is correct you have a major problem.
There remains the small possibility that your phono amp is faulty. However I would bet heavily on the magnets is that replacement stylus have and probably still are loosing their magnetism.
