I think I've found a way to make these Monoprices sit nicely in my ears.
Instead of using the stock tips or medium size Sony tips, I tried using the large Sony tips.
The large tips don't fit me at all: They're far too large for/wide my ear canals... So instead of getting deep insertion, I've fit them like I would an earbud [not canalphones]. That way they just sit on the outer canal, and when a seal is achieved, the housing needn't be touching my ears... which is nice, I think.
The problem I had with the Monoprices' fit is that, (a) I can always feel the earphones are on (the housing touches my ear.. in an uncomfortable way), and (b) the earphones always fall out of my ears. The large Sony tips alleviate both problems rather well. I can easily forget I have the earphones on (albeit the microphonics are always ever-present), and the earphones stay on and the tips seal well.
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There are little to none audible differences in sound... but I can't be sure. The differences in sound are far less significant than that of comfort for sure.
I do feel that the sound does get better by a small margin, though. I've always felt a slight oddness in the sound of the Monoprices (which I described as "perpetually unthick" in another thread), and that's lessened somewhat... I guess. It's hard to be sure. After all, that "oddness" I was hearing could be the effect of a poor seal, right?
Don't get the large Sony tips for "better" sound, that's my recommendation. The sound remains the same for the most part. BUT, the comfort/fit/seal is far better now.
With the stock tips (and medium size Sony tips), getting the 8320s to seal was a salty pain in the places I wouldn't like salty pains.