Headphone setup - dual setup w/ speakers

I'm venturing off into the world of headphones with Sennheiser 650s.  I'm wondering what the optimal signal chain should given my current setup:

Modded Technics 1200 -> Lounge Mkiii -> Pioneer VSX-1120k -> Kef LS50.  

I'll likely start with a 50/50 time split in terms of listening sessions.  This will never be a dedicated headphone setup.  I'd like flexibility in the long run - something that doesn't have me swapping cables before I sit down for a listening session.  I'm most certain that I can plug the 650s directly into the Pioneer, but in order to get the best reproduction, I think I need to run a dedicated headphone amp.  

Any suggestions on what to do?  Are there any headphone amps that have a passthrough?  I think this might be what I'm after.