My previous best upgrade was to the deHavilland UltraVerve tube preamplifier. At the time I was a dealer for Art Audio, Joule Electra, Atmosphere and had pre amps from all the above. In the hopes of finding a less expensive preamp I ordered an UltraVerve from Kara Chaffee. Well I got more then I expected. That little $2500 pre out performed all the above. What an eye opener that was.
I think that was in 2006.