Been a question for me, too. Best case solution is to use the tape loop, as that (often) bypasses the volume pot on the preamp and allows you to have only one in the circuit (the one on the headphone amp). Folks can certainly disagree whether having the extra volume pot in the circuit will noticably degrade the sound quality, but on the theory that less is more, there is really no reason to put it in there if you don't need to. If that isn't an option (as it hasn't been for me since I switched to a preamp that doesn't have a tape loop) you can also use a second line level out from the preamp. This does, however, create the redundancy mentioned above--namely the second volume pot. My answer to this has been to dial in the preamp at rougly zero gain (on the theory that it will then have the least effect on the signal, positive or negative) and then to control the volume level at the headphone amp. To do as you suggested (keep the volume on the pre low and then use the headphone amp to turn it back up again) is likely not the best idea when it comes to fidelity (though it will certainly "work"). By turning down the volume at the preamp (thus attenuating the signal to a minimum) and then taking that reduced signal and amplifying it back up again at the second volume pot you are effectively creating the maximum possibility of altering or degrading you signal given the setup. (Hope this isn't confusing). Short answer is, sure, any of the suggestions will work fine -- some may sound better than others, though. Try it and see what sound best.
