iMac vs DacMagic?

I've got a Dell Inspiron 14. I thought, perhaps, with an external sound card, such as the Creative Sound Blaster X-FI HD, with 24/96 USB to S/PDIF, I could shoot direct to the S/PDIF DVD input on my Sony TA-E9000ES, expecting, of course, better sound. Well, after I purchased the device and not getting any better sound with it, I got to thinking again, I wondered, what sort of sound card did I just bypass? Upon some research, I discovered my Dell's internal sound processor is an IDT HD Azalia type, which  processes at 24/192. In other words, my Dell's sound card is real good; in fact, I can hear no difference between it through  RCA's  to the Sony, or S/PDIF from Sound Blaster to Sony. I just wasted some money, it appears, buying the Sound Blaster. What I'm getting at here is that you may not hear anything better, given that your MAC's sound processor is, pretty much, as good as it gets, today. The only products that seem to make any sense are DAC's which have higher rez than those found in the computer. I now wonder what's the point of many of these devices.

If you're plugging the amp straight into the Mac, then you're most likely double amping the speakers, which can lead to nasty clipping, distortion and other random audio effects that won't help sound quality. A proper DAC will allow a line-level signal to be fed to the amp, which should at the very least reduce the chances of this happening.

And the sound processor has nothing to do with this sort of problem. The issue present is the analog implementation of the sound output, in which the DAC chip (digital only) that's being implemented in the Mac or Dell will almost have no noticeable bearing on the analog output of either or any.

Double amping? If you mean gain controls, you've got plenty of 'em throughout the chain: software,  computer sound volume, DAC possibly , as well as at the control amp. It is what it is, but not a problem, usually, even in shared mode. In my system, full gain on the computer devices, iTunes, and Sound Blaster-all the way to the preamp works for the best sound from the preamp to amp. At the power amp, I also have full gain. The preamp's volume control, at oodb, still starts at a silent level, so I've got a real low niose floor. I guess there could be a clipping problem, but, I've only experienced it when recording, or playing back from a balanced line source to an unbalanced device, or from unbalanced to balanced. In either event, gain at one, or both devices can be adjusted to yield the most dynamic range without clipping. Of course, in digital to digital movement, there's not likely to be a problem of mismatching line levels since the standard between consumer and professional audio f S/PDIF is alike.