If your cassette deck has a 'line' out (like most home stereo decks do) I'd 
suggest using it, as opposed to the headphone output. The signal at the 
headphone jack is pretty hot (not necessarily bad), but it also carries all 
of the noise of the output section along with it. Use the 'line' out to 
help maintain your S/N ratio.

Also,  don't discount the sonic performance of a cassette tape source. 
Assuming a well-adjusted deck with good heads, and a quality medium, 
cassette can be a very high fidelity source. Too much blame has been laid 
on cassettes, due to the way the tapes (and players) get treated in 
automotive installations. A quality cassette is quite capable of standing 
up to, or even outperforming a CD.

Bill