Headphones $600 or less

You don't mention your power source, but if adequate, the Hifiman HE560s might be a nice fit. They are a little less efficient than say the Audeze LCD 2s but can be had new for less than $450.00 and have seen them used for half of that, by the way they are open backed. Enjoy the music

Pre amp I’ll be using is a Rouge Metis turntable is a Rega RP 3 and have a Rega Apollo CD player. 

My favorite headphones that I've owned in that price range are the Drop Focal Elex (exactly $600).  I also own Hifiman HE560s (these are hard to drive), Audeze LCD-2 (you might find some used at that price), Audeze LCD-XC, as well as some high end headphones like Audeze LCD-4, Sony MDR-Z1R, and Sennheiser HD800.  The Elex are comfortable, sound very neutral and detailed, and are well made.  You can also find a lot of options for different cables.

Be careful - I just saw an interview with Pete Townsend (the Who) who said he lost his hearing from listening too loudly at home with headphones, not from the sound on stage.  I don't know if he's right, but that's what he said.  That said, the Oppo PM-3 phones are very good and can be gotten at decent prices in many cases, despite the high asking prices for Oppo 205 DVD players.  I've also seen some PM-2's around, which are even nicer.  They are very efficient, so they can be driven by low-powered amps.  I am partial to my Grado 1000 open-back phones, but the Oppos were really the first closed backs I liked.