A receiver or pre-amp will work. Just be sure to find one with a set of RCA "line out" or "pre-amp out", and run those to your Crack. The only problem with this set up is that your speakers will also be running.
I have been completing my Crack (finally found the last loose connection!) and have been running it off of my pre-amp. This weekend, since my Crack is now crackle and pop free, it will replace the small SS headphone amp that has been holding its place. I will run the Crack directly off of my DAC, which has balanced XLR out to my pre-amp, and a set of RCA which will go to the Crack. I leave the DAC running 24/7, and when I have the set up done, can either turn on my Crack and do headphones, or turn on the rest of the speaker set. Now, if I can just remember to power off the Crack when I am done...
The only problem with my eventual setup will be that I my only source will be the DAC, and currently, with output from the Pre-Amp, I can choose from DAC/Vinyl/SACD player. I tend to use headphones mainly in the morning, to avoid waking my family though, so I am not seeing it as a problem so far.
I like the splitter solution mentioned above though, and turn on which ever amp you will be listening to, fairly cheap, and simple to use solution, without a downside.