Hey David, MJMP can no doubt help with the MP1 noise mod details, but as he said previously, the amount of improvement is dependent on the starting point (current state of components) so hard to put a % measure. If, as you say the unit is stock, there will be benefit from doing it and it will also enhance the sound.
Age of and choice of tubes will also play a part. If the tubes are also original, then it's very likely time to change them, basically, you need 2 high gain, low noise, low microphonic 12AX7 tubes. ADA recommended a tube change every 2 years for the MP1 but this obviously is dependent on use. This post (http://adadepot.com/index.php?topic=300.0) is one of the many here relating to tubes and tube choice (this BTW is a very subjective area). Actually a change of tubes may go a ways to solving your problem(s)
As the MP1 is a tube preamp, I don't think a DC power supply (like Alesis use with Quadverb etc) would not be sufficient as the tubes need (AC?)heater current etc which comes from the power transformer. However, a very worthwhile mod is the MP1 MDRT transformer replacement (http://www.marshalljmpmodshop.net/miscellaneous_parts_and_replacement_tubes.htm). A decent power conditioner (e.g http://www.furmansound.com/product.php?div=03&id=PL-PRODMCE) will help (as you found on the 1 in 30 outlets
). The link is to the 220-240v version that I'm interested in, the same is also available in 120v. From what I've found out recently, the Furmans have a good reputation.
Cheers Richard