Unlistenable someone posts? Don't understand the need for such sweeping hyperbolic posturing.
The internal drivers of the 27" iMac are perfectly respectable for monitor speakers. As someone mentioned they are above average for monitor, of course not equal to standalone hi-fi, and assuming you don't push the amps to clipping.
I would say more analytically they deliver clean, flat response from the upper register on down through about lower midrange/upper bass, any lower which is physically impossible due to enclosure volume restriction. That said I do rock one of the new Tango soundbars for my iMac, which has the sonic effect of slipping in that missing lower midrange band and gives an extra punch especially to electric bass. Then I loop in a small 4" subwoofer to fill out the basement and am quite happy, not in the sense of "fixing" the oem speakers, but augmenting them. If all's I had was the stock drivers I would be just fine still, and certainly do not consider them unlistenable.