i have the MV-100, and like it, but the gain is stepped at 20-40-60 dB and running into a d100 that is sometimes too hot with my Oktavas, which put out a weaker signal than the 390s. you cannot run the d100 under 4 on the input dial or it will brickwall.
if you do choose to get an MV-100 definitely pick up a pair of attenuators!
i like the preamp, sounds better than PS2 - LT's did to my ears, and it's built very solid, can run for 7-8 hours on 2 9V's putting out a full 48V, i imagine it will run much longer at the voltage 390s require...