I'm not sure of the ins and outs of the bits and bytes, but I've used Spin Doctor quite a bit, and it works great. What I've done is plug my tape deck into the mic jack of my PowerBook (an 1/8th inch mini-plug to stereo RCA cable comes in the box). There are controls for gain and some basic filters for reducing pop and hiss, adding bass, etc. The instructions that come with it are quite good.
I'm not sure if it, itself, is worth $80 (there are share-ware apps that'll do the same thing), but the entire Toast Titanium suite of apps is more than worth the $80--make sure you download v. 5.1.