High-end audio is by nature a very limited market segment to begin with and the discussions above about wanting to hear a difference when one really can't be heard or simply doesn't exist with respect to cables also applies to the amps. I would wager that most of your customers claiming to hear differences actually can't under a double blind, level matched, etc. test which would further support any nay sayers who believe two solid state amps performing within spec can't be distinguished from one another. What is measurable can not necessarily be heard by the human ear so yes technologically amp A may be "better" than amp b, lets say SST vs cube but that's not necessarily going to translate into a listeners reality when un-swayed by word of mouth, positive reviews, newer is better, more expensive is better, heavier is better, etc., etc.
At the end of the day you have to choose to accept or dismiss it and just continue to do what you do and continue on making your products as good as you can.