I run mine into an Apogee Mini-MP preamp. I have an extra Apogee if you want it... $300.
Also, be aware the OM1 can put out a high signal for loud, bass-heavy music, so make sure your preamp can handle it otherwise invest in a few attenuators. For example, I need to use attenuators to run OM1's direct into a Tascam DR70D.