So I ran the 391(s) with both phantom power/transformers into mod sbm1 and mp2 into mod sbm1 and I can say that the MP2 actually helped define the bass better than the phantom/transformers did. My belief, as stated earlier, is that these mics are bass heavy in some environments as a characteristic. That being said, I currently run Gefells>MP2>Mod SBM1 and the bass is incredible imho...well defined not overpowering, leading me to believe that my original belief on the characteristics of the 391(3)s weas about right.
Definitely dont let that stop you from trying a different preamp, because it absolutely may help...just throwing a different view out there, from a former user of these mics...which by the way, best mics at that price range imo!