The dealer I've worked with the most over the years just got in Conrad Johnson's new CT-6 preamplifier which is in about the same price class as the deHavilland Mercury 2 (the CJ costs $4,500). He encouraged me to take it home over a Sunday and report back my impressions. It was embarrassing - for CJ. By comparison to the Mercury 2, the CJ was just dull. The Mercury surpassed the CJ on every aspect of the sound - treble, midrange, bass, harmonic complexity, soundstage, and naturalness.
Perhaps an explanation for why the Mercury 2 sounds so good in my system is that my amplifiers are custom-made monoblocks using 845 tubes. deHavilland has specialized in producing 845-based amplifiers, and this preamp was specifically designed to go with them. 845s are big tubes that run at very high current levels, making huge demands on their power supplies. Perhaps designing the preamp to go with 845 amps results in a real mismatch with at least some other amps. That's the only thing I can think of that could explain why impressions of it range from "best tube preamplifier on the planet" to "failed miserably." All the more reason to try-before-you-buy.
Perhaps an explanation for why the Mercury 2 sounds so good in my system is that my amplifiers are custom-made monoblocks using 845 tubes. deHavilland has specialized in producing 845-based amplifiers, and this preamp was specifically designed to go with them. 845s are big tubes that run at very high current levels, making huge demands on their power supplies. Perhaps designing the preamp to go with 845 amps results in a real mismatch with at least some other amps. That's the only thing I can think of that could explain why impressions of it range from "best tube preamplifier on the planet" to "failed miserably." All the more reason to try-before-you-buy.