Well, I've found out that the Klipsch ProMedia 2.1 THX-Certfied speakers don't have a digital input, which to some apparently signals that the response if connected to a computer, or portable player, is weaker.
But that is only a partial insight. It so happens that, to compensate for lower output voltage in portable devices, and in computers, the stereo input in these speakers (mixed with the main input) includes a +6 dB boost. This boost, and the 24" iMac's beefier 24-watt digital amplifier, not to mention the excellent quality of these Klipsch, go a long way to ease any worries about power, volume, or sound quality. In fact, other considerations carry more weight: quality of the music (thrash in - thrash out) so higher bit-rate encoding probably has much more of an effect on sound quality, and of course, type of music (this system is excellent for music lovers and has been designed with such in mind). Other things come to mind... placing of speakers, room, etc.
So, the answer seems to be, at least for me and my own tastes in music, in listening, and whatever hearing loss I have, that the fact that the Klipsch ProMedia 2.1 THX certified speakers don't have a digital input means very little in terms of getting excellent quality sound connected to your computer.