Groove Tubes Gold Series GT-6L6-S Matched Power Tubes Medium (4-7 GT Rating) Duet guitar amp tubes are Slovakian-made with high power and very aggressive dynamic tone...punchy, but still sweet. New GT design with more power and heavy duty components. High power type with more highs and lows than other 6L6 types. Country of origin: Slovakia Soft, medium, and hard style descriptions refer to break up modes of the tube for an overdriven guitar signal. Soft-rated tubes break up easier and have the least headroom, yielding a heavier, harder distortion. In comparison, hard-rated tubes break up easier and have the least headroom, yielding a heavier, harder distortion, while medium-rated tubes fall in between.Country of origin: Slovakia