I have serial number 14 with the six tubes in a v-formation. The unit is amazing. I am curt comparing it to going directly from my modified Sonica DAC into my Decware Mystery amp (ZMA) and I think the preamp adds depth and sound stage and makes digital sources less harsh with my Focal speakers. I bought the ZTPre with all options so I have XLR and phono. I use phono for my Chinook phono amp and the XLRs for the DAC and another phono amp. The preamp really shines with phono. The unit is about as close to no preamp as you can get as far as coloration, but with awesome gain.
What amp are you using?