I had a 1987X that I shouldn't have sold, and I installed the "Rich" PPI MV in it. It worked. However, it changed the tone of the amp. It was much more mid-heavy and the highs and lows were attenuated. I left it in for a year and then returned it to stock and got the great sound back. I bought a Dr. Z Airbrake and kept the tone with better control over the volume. I've tried other PPI MVs in Marshalls and haven't liked any of them. To my ears, it's worth spending the extra money on a sonically-transparent attenuator (the Dr. Z and THD attenuators are both particularly good with the 1987X). If you're attenuating way down to play at non-guitarist-friendly volumes, your amp will sound less like itself because the speakers will react differently. Lower-sensitivity speakers and a good attenuator are a good approach as they'll work together to rein the volume in.
There is no substitute for Sound Pressure Level