Are you presently using a MM cartridge that provides a higher output? If so, let us know its output rating, and give us some idea of the highest position at which you normally set your volume control. For example, if it is a rotary control its minimum position may be 7 o'clock; its maximum position may be 5 o'clock, and the highest position you normally set it to when playing records may be 12 o'clock.
Then, with no music playing, try to determine how much further you can turn up the volume control (e.g., what clock position you can turn it to) before hiss levels become objectionable, or you reach the maximum possible setting (whichever occurs first).
You should certainly talk to Keith, but providing us (and perhaps him) with that information may also be helpful (if, as I say, you are presently using a higher output MM).
Regards,
-- Al