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.
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.