headphone amp setup question

Here's the situation. My First Sound Presence preamp doesn't have a headphone jack, and the only way to listen is through my cassette deck jack. Am thinking about getting a good headphone amp. My question is this: to connect the amp, do I have to use the tape pre-out jacks, or can I use the unused pair of outputs from the preamp and just set the preamp volume controls low and use the head amp control to adjust the sound level. Thanks. JimL