Sometimes if you using cheap interconnects such as a lamp cord you will have a problem telling a diffrence. Maybe you need to get a better amp to hear a diffrence (no offense) Maybe they are equal in quality and also share the same characteristics as one another?
new amp vs old amp
i bought a acurus a150,i also have a dynaco st120,very different amps!one circa 1970 the other 1990'ish.ive done brief a/b testing,swapping them back and forth.my problem,i hear very little difference.why?do i need to listen longer?ive always though i had a good ear,i can here light notes and quiet backround instruments in music that most people cant!!!whats wrong