Fivefasts, the recommended load capacitance for the M91ED ("N91ED" is its stylus assembly) is 400 to 500 pf. I would expect that under typical circumstances the total capacitance of the turntable wiring, the phono cable, and the input capacitance of a preamp or separate phono stage will be in the vicinity of perhaps 200 to 250 pf or so, and sometimes significantly less.
Some Googling will reveal numerous comments by users to the effect that loading the M91ED with too little capacitance will have adverse effects in the treble region, and specifically that it will weaken the brightness region.
Also, my perception has been that generally speaking Shure cartridges are not known for their brightness. And the graphs I linked to earlier, for the frequency response of the M97xE, are certainly consistent with that. It could be that the cartridge is just not to your liking, and your present system configuration is not compensating for that to the extent it was previously.
Regards,
-- Al