There are MANY things that can cause the effect you are describing - and unfortunately one of them is age (I know about this one!) As we age one of the first things that often happens is high frequency hearing loss and/or sensitivity. Another is your listening environment. Heavy fabrics (drapes, carpet, upholstery, etc) can "soak" up high frequencies - on stop them from bouncing around. Generally, this is a good thing as hard surfaces can reflect sounds and mess up the overall sound of a good system.
Different amps (receivers, etc) have different settings for the boost or cut from the bass or treble controls - its a designers choice. The amount of boost or cut varies - as well as the center (target) frequency for which the control is set for. So it is often difficult to do a direct comparison of different equipment with the settings at anything other than "flat" (no boost or cut)
The age of the equipment can be a factor, since both tubes and solid state components age and deteriorate over time, which can lead to signal degradation (especially at both ends of the audible spectrum)
So unfortunately there is no simple answer to your question.
My advice: If the sound is "better" to you with the treble boosted - then boost it and enjoy!
Life is too short to worry that some audophile thinks you must listen to your speakers with you amp's tone controls set to "flat"
