Sonic Frontiers SFL 1 Sig. Edition Advice

I have a SFL 1 SE pre-amp. I would like to use it as a headphone amp with my Sennheiser HD 600's, but there is no headphone jack for this purpose on the pre-amp. Could I simply get an adapter at Radio Shack that would allow me to connect my headphones to the RCA outputs on the back? Thanks for this much appreciated advise.
Sorry, if this doesn't answer your question, but, you might want to consider selling your SFL 1 and moving on to a Sonic Frontiers Line 1. The Line 1 has pretty neat headphone compatability built in.
I really like that SFL-1 Sig. You might contact Parts Connexion. They are the old SF/Assemblage/Parts Connection folks. They might be able to add the headphone jack.