Steve's notes for the mk II indicated it needed 3-4V - which the mk III does not - but I was never sure whether he meant rms or peak.
Certainly the Taboo sounded better with the Eastern Electric MiniMax's 2.5V rms in tube mode, but best of all (fed direct) is from my BMC PureDAC at full output (have no idea what the Vout is but it's certainly more than 2V rms). Of course the PureDAC's abilities compared to my other dacs have a part in this.
Regardless, the Taboo in any form is a heck of an amp with my LCD2 and (recently) LCD3F, also great with my 92dB/1W/1m speakers. Equally good is the CSP2 (now CSP3 as Eric noted above) with Beyer T1. Very dynamic, transparent amps with lots of tube-rolling possibilities.