It's the devices (computers, iPhones), but also the earbuds and little speakers - and the people using them don't really care about the crappy quality.
The trick to good mixing is that it translates well onto
any playback device. To do this, you need a monitoring system that gives you an unbiased sound, acoustic treatment in the room, and experience knowing what music sounds like on that system/in that room - reference mixes.
I listen to lots of other people's mixes on my system, and read the comments from others to what they hear on these mixes - as well as what they hear on my mixes. I burn CDs and play them in my truck, on my home stereo and on the 5.1 video system, too. Listening with headphones is a good way to check a mix, too (but not to do the mix itself because you lose stereo perspective).
Jookey - that Pioneer system is probably not giving you a flat response at all, even with the EQ flat, as home stereo speakers have always been designed to accentuate some part of the spectrum- the bass and highs most often, but some for the mids (those who love guitar rock want that). When I worked at a stereo store in the early 80s, I learned which speakers had the more-flat sound (KLH, Advent, Boston Acoustic, for example) - but these weren't the big sellers.
Mixing for different mediums (CD vs vinyl) has come more to the forefront with the resurgence of vinyl with audiophiles. And its needed - you have to keep the bass volumes lower on vinyl or else you risk causing the needle to jump the groove.
Computers 'accentuating the 1MHz' range? No, it's the speakers. 3" (and smaller) speakers have no bottom end - anything below 100Hz is not reproduced at all, all you hear are the harmonics. And the 1MHz range is where those small speakers work most efficiently. Earbuds and headphones tend to have louder bass - proximity effect to the eardrums.
