I have the s300 and had the same initial impressions of the vocals being overly recessed, almost to the extent it had an echo. This amp had an overly long break in time as compared to anything else I've heard in my system. After about a week of being powered on continually this anamoly started to disappear and didn't fully disappear until it was played heavily for several weeks. Bel Canto says 100 hours of power on but I think it is on the low side. Sit back and give it a chance it gets much better. The s300 I have is the Underwood mod maybe this took longer with the added caps.
Glenn
Glenn