using two outputs from a dac?

Thanks, Erik. I'm afraid I'm pretty dense about specs. The (tube) preamp input impedance is 50k but the headphone amp is 10k. Is that a problem? I should add that I'm not trying to run them at the same time (obviously?!). I should also say that this isn't a permanent set-up; the headphone amp will get its own dedicated system soon enough. 

Thank you both so much. @erik_squires--that's reassuring, at least in the sense that I'm not going to fry (or blow up!) anything. @mksun, very bright and pretty cool to have such a bespoke cable. If this setup becomes permanent, I'll consider one for myself.

Thanks, all. My DAC doesn't have a volume control but I can certainly turn off my preamp or switch the input on it.

Thanks again!

Thanks, @desktopguy. I'm going to follow up and get the manufacturer's recommendation. I appreciate the advice!

Update: Bryston says it's fine to run them both. Thanks again for all the help!