Harp is one of the most difficult instruments to record well.  (Guitar is amazingly simple in comparison) My wife has 3 harps (all purchased new, and all in excellent condition) & I can only get good recordings on one of them.  In general, smaller harps are easier to record than the larger harps.  I started with the same preamp you are using, and it was not until I switched to a better preamp that I started getting good recordings.  The real secret, even beyond the mics, is the mic positioning.  Currently I am using 1 Blueberry mic just in front of the harpist shoulder, angled towards the body of the harp, and 1 Neuman in the back aimed towards the sound holes.  Read Harvey's thread on mics, as that should help with the conceptual part of this.  Whatever mic you use, be prepared to spend inordinate amounts of time with the mic positioning, as this will make more difference than the mic choice.  
If I was trying to do this on the cheap, I think I would get several of the cheap Chinese tube mics that are discussed here, and then invest in a better preamp.