What @Sparky2 said. It does sound odd to many causal players, but techs....at least in my circles...use it all the time. Clean tones may sound great withrelatively "neutral" settings a in some amp, but if you need to bring up the highs and/or upper-mids to "cut" in a live situation the "clean tone" in that situation may be brittle. Or an amp may sound great when set to "cut" may be muddy and unclear when adjusted for a jazz guitar situation. *OR* an amp may sound wonderful loaded with reverb for "surf" but without reverb sounds dead and clinical. Orrrr - without reverb a combo amp may be great, but its reverb tones are sharp, metallic and "clanky". Simply - there are multiple clean tones in ANY amp, and when discussed as a "whole" they are "cleans". So from my standpoint, even if a player is a one-style, one sound player I will try, when performing service (which almost always includes some "voicing" - at least in my case), to get a good, full balance just in case the amp has to be used by someone else. But only after clearing it with the client and assuring them I won't compromise *their* specific needs. Does that explain why it's often used in a plural sense?