If you are using the guitar for recording, the piezo pickup alone won't sound very good IMHO. They are good for live use; not real prone to feedback (as acoustic pickups go) and have a lively, zingy character that cuts thru a live mix, but they are very harsh sounding for recording, no matter what preamp or exact model of under-saddle pickup. It might sound better to you thru a guitar amp, because the signal is being "softened" a little. For recording and getting a true acoustic sound, you will need a good condenser mic - read up on placement, EQ, etc. If you don't have the cash for that, try experimenting with compression and EQ to try and tame that harshness...you might also try mixing in a little of whatever mic you have laying around; an SM57, SM58, etc. with the piezo signal.
Apart from that, I am a little puzzled by your comment about it having something like an echo or reverb-like quality...I wonder if somethng could be wrong with that preamp ?? The output wire from the piezo should have a 3/32" (or is it 1/8" ??) plug on it, you could unplug that from the preamp and try running it to your board directly to see what it sounds like. The signal will be weak (un-pre-amped), but the trim/gain knob on the board should bring it up loud enough. If it sounds way better than with the preamp, you'll have to get it fixed or replaced. I have never messed with a 12 string that had a piezo, but I wonder if the reverbish quality might just be due to sympathetic vibrations from all those strings?