I remember when I was a young adult, I had speakers that were built by a friend of mine. He didn't really know what he was doing and they didn't sound very good - hardly audiophile. At the time I had no money for speakers and was happy to have them.
A few years later I bought my own pair of speakers. I purchased them from a good audio shop and didn't really audition the. I simply took the advice of a salesman (whom I happened to know and trust). He warned me that they were considered pretty flat and it might take me a while to get used to them.
He was right. At first I thought they were pretty bland sounding. I listened to music as a whole back then, not individual instruments. I just cared that there was bass and that they played Deep Purple and other classic rock nice and loud. Later when I started playing bass I started to appreciate my flat speakers as I could follow bass guitar lines. I still like a solid foundation, but if it's muddy or boomy in any way I'm not very engaged.
All this to say that if you're attuned to a certain sound, especially if it has emphasis in certain frequency ranges - or even distortion, it can take you a while to adjust when you hear a system without those characteristics.
After all, think of the occasional young drivers who pass by with a zillion subs in their car and super hot treble - i.e. anything but a flat frequency response. We wouldn't say that their inability to enjoy a flat presentation was the fault of the gear, would we?