Some basic things to check for are:
Samplng rate - you want at least 44.1Khz
word length/bit depth - at least 16 bit.
This is on the way in, of course.
When bouncing down, what are the settings you're using? .WAV files can be any sample rate/bit depth can't they? And AACs are lossy, so you can definitely crunch up the top end by using low bit-rates. You might try doing a full-sized WAV file (CD-quality or better) if you aren't already.
Those checks done, I would check all cables to make sure the signal isn't getting mangled on the way into your computer. 25' is long enough to get some audible loss from the phono pre to your computer input, especially going RCA to 1/8". Loss of high-frequency information is the usual result, though.
Try monitoring the sound from the computer as you record (headphone out jack from the mac?). Do you hear the sibilance on the way in? If so, that long cable would become the #1 suspect for causing signal damage. Perhaps it's the length, the connectors, or some combination of the two.
If those don't cover what's going on, and there's really no extra processing going on (EQ, "mastering" compression, etc), then I'm at a loss. Good luck again!