750 are closed, 2500 are open.
If isolation is a factor, the choise is easy.
Sound wise I find them to be different in the following aspects:
750 is more airy, lighter and sometimes has an echo/reverb that bloats the bass. More coloured (you can like it or not).
2500 is more full in sound, has no reverb or echo, but also sounds somewhat "flatter" . More neutral.
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FS/FT: Koss ESP9
$150 amp vs. $250 amp?