Peter Drefahl of Drefahl Audio in Germany converted several Gefell MV692's to 48V phantom powering for me 8-10 years ago. The Gefell PM 750 is very similar to the MV 692, the PM 750 uses a large exterior housing to protect the microphone capsule which is not used on the MV692.
Peter also replaced some of the older, noisy "East German" electronic components in the Gefell MV692 amplifiers at the same time that he did the P48 conversion. I think that you would want to replace those noisy components in the PM 750 too.
Peter is a well respected microphone technician and does excellent work!! I can't recommend him highly enough. The downside for you if you don't live in Germany is that you'll have to ship the microphones to Peter in Germany.
Peter speaks English and will be happy to provide you with an estimate for the 48V conversion for your mics. Here is the ink to his web site.
http://www.drefahlaudio.com/