For a while, I ran the 391's into a ps-2 and then to a mod SBM-1. usually, the PS-2 phantom power supply
will supply 48V. however, I do know that they made 9V version, so you might have that. regardless, you're question is about gain. The cheapest way to do this is with line transformers. I got a pair at Radio shack for maybe $20. I know Hosa makes some nice ones as well, probably slightly more expensive, but probably better transformers as well. I think I calculated that my RS transformers provided about 12dB of gain. here's a link to a Karl Denson show from the archive that I recorded with this method:
http://www.archive.org/audio/etree-details-db.php?id=4647After a few months of running this, I got a Beyer Dynamic MV-100, for phantom power and gain. and I didn't like it as much as the ps-2 > transformers, so I went back to the transformers. the transformers are definitely a nice cheap option for clean gain. I'm sorry I can't reccomend the exact model number for the hosa ones, but I'm sure if you did a search on here for "line transformers" you could find it pretty easily.
hope this helps.