Hi dgozalie,
I believe it is fine to leave headphones plugged in to one's headphone amp all the time, and have done it consistently for about 15 years with my SEX amp and Sennheiser HD600's with no ill effects. One recent change has been to add a headphone stand, which I consider a very convenient addition.
The only reason I ever unplug is to switch headphones, move the setup, or to bypass the headphones to the speaker outputs (not an option on your Crack amp) none of which happen often. I'd say leave the cans plugged in with no worries.