There are a number of rackmount preamp options. The Tech21 Sansamp RPM and RBI, seem very popular (plus the upcoming VT version). Jule Amps Monique also gets lots of love, deservedly so. Disclaimer: I have an RPM and an RBI. Which are the other preamps that are in the must-hear category? Looking for rackable preamps that are still in production and available. Also, can the preamp be described well either by category or perhaps artist? I know that's a subjective and difficult request at best. I'll leave it open-ended in the hope that a broad and useful list is built by your suggestions. Thanks in advance.