I've owned Quicksilver 300B SET amps for about six years and love them. I have not heard Mastersound so cannot compare. The Quicksilver amps have huge power supplies and are wired point-to-point by hand. Bottom end is excellent, especially for a 300B amp. Midrange is superb. I'm using WE 300B's and newer Svets on occasion. WE's certainly best the Svets in overall musicality and just seem smoother, for lack of a better descriptor. They are rarely seen used, and Mike Sanders does not advertise them on his website, but did show them at RMAF 07, where they sounded astoundingly good with a modified horn speaker, which I cannot recall the manufacturer. He builds them on demand only - you will not find them on his website. I believe the price is $3000 with no tubes. $3200 with (not sure which tubes he provides). He also makes a KT88 SET amp which I have not heard personally, but a close friend who owns several of his amps including that one speaks highly of it. I believe it was also reviewed in one of the rags. That amp is on the website.