I think i have experienced this many times before, especially when i was younger, under various conditions, but difficult to clearly pinpoint 1) what is it and 2) what causes it.
Just a little searching on this question turned up various results that somewhat mirrors #1 & #2 uncertainties in various contexts; some call it ear fatigue, some call it listening fatigue, my quick search did not find psychoacoustic but i did mostly gloss over the wikipedia entry. One web site attribute this to THD. Some people basically attribute this fuzzy IT to too much listening to anything at loud or lesser volume for too long. Audio engineers / sound mixers are apparently prone to ear/listening fatigue due to regularly listening to sound that at times is loud, day in & out and for which short breaks OR longer periods of rest might help.
Audio / music enthusiasts - such as you/we are - might also experience this under various circumstances such as extended listening via speakers/monitors while other people might notice experiencing this only using headphones. Some say its partly due to certain frequencies - some say high freqs, others say low freqs - and others claim it is the make/model of headphones yet others claim it is the design - weight or clamping force or closed-back versus open. I would also add from my own experience that musical content - like the local FM hits station - played loud, for hours, through a crappy stereo/receiver/speakers set up during a Saturday night kegger in a buddy’s living room.
As such, there seems to be multiple variables in play. Your situation is another good example; you find a particular VST played through an ipad quickly disturbs your listening experiencing while you find no issues playing any of most other VSTs for a long time as long as the volume is set to a correct value. This leaves at least a few unmentioned variables; how do you determine whatever is your correct volume and is it played through the same ipad, or other computer, via monitors, headphones or earbuds?