And I Quote...
Fast Recovery Diodes (FREDs) and Schottky DiodesCompared to silicon diodes, FREDs have a full and airy sound. The bass is tightened up, and the harsh edge in the mids and highs almost disappears. Details in the music start to come out with greater clarity than ever before. FREDs sound as close to a tube rectifier as one can get, while not requiring additional power to run a filament or heater.
Most modern audio equipment including amps, preamps, CD players, etc will be improved significantly by installing FREDs. Good vintage hi-fi candidates for upgrading to FREDs include: Fisher 400, 500, 800 receivers, Fisher tube integrated amps with silicon rectifiers, EICO HF-87 and HF-89, Harman-Kardon Citation II and V, H. H. Scott 340B receivers, Sherwood integrated amps and receivers, McIntosh amplifiers, preamps and tuners, Marantz amplifiers and preamps, etc. Heck, they will even make most of your solid-state gear sound much better.
http://www.vacuumtube.com/Products.htm##Fast%20Recovery%20DiodesThink what they colud do in a transmitter PSU! Actually come to think of it I'm half way there as I use schottky diodes as the damper diode in my PDM modulator - no wonder it sounds good!
Ian VK3KRI