I have the same issue w/ my integrated amp. I decided to use the Tape outputs from the Krell to keep the integrated amp's volume separate from my headphone amp's volume. By doing this you can turn off the sound from your preamp and control the headphone's volume independently for discrete listening. The only disadvantage is no remote control over the headphone's volume adjustment.
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