Z, no, there isn't any principle of audio technology which would lead to an amplifier with a higher maximum output capacity producing better sound even at low or moderate levels which were well within the capacity of an amplifier with a lower max. What the salesmen at the stores told you on this subject would fall into the voodoo audio category. Similar claims are made fairly often, but they have no factual basis and have failed when put to a properly controlled blind listening test. Clean is clean, regardless of whether the maximum output capability is 10 watts or a thousand watts.
Your 1800 is quite powerful(and of course is "good quality")and may very well be entirely sufficient for your needs, depending on how high of an average listening level that you use and how wide the dynamic range of the source material is.
