Upgrade recommendation. Richard Gray or Mcintosh.

I'd spend my money on the MC42 unless you have power problems the Monster causes or can't cure. After you get the new pre-amp you can check your power situation again.

Upgrade the preamp, no question. Unless your Monster is malfunctioning (and then I'd just unplug it), you're talking about a couple of levels upgrade on a component that every signal in your system runs through and which is most likely to interact even better with your amplifier than what you've got; while power conditioners/regenerators do make audible differences in a system, I have never heard one that would make a difference as big as what you're describing with respect to the preamp upgrade.

Preamp - but consider the C2200 over the C42. Skip

You will be impressed with the 42.Do you have the 352 plugged directly into the wall?