My preamp is a bone stock ZTPRE. I use it to drive my Signature Mono Blocks. Because it is so transparent I often use it to drive other amps like the Original Zen, Select, or Integrated amp. Even though these amplifiers are designed to be directly driven by a source, I often just leave the ZTPRE hooked up all the time.
You can get more information on the ZTPRE here.