Preamp headphone amplifier

Velocityofhue    Thanks for the reply. I like mostly 60's, 70's , 80's rock not much hard rock unless driving. More like Steely Dan, Boston, Ambrosia,  etc. Lots of music with instruments and harmonies. My current setup is the 526 preamp, Levinson 534 amp and Revel F228be speakers with all Cardas Clear cables so I like hearing the details and nuances of the recording.

Headphone amps in audio amps a largely an afterthought and really bad.  You have nice headphones, so don’t starve them with an insufficient amp.  This is a good one IMHO.  

https://shenzhenaudio.com/products/singxer-sa-1-fully-balanced-class-a-pre-amplifier-headphone-amplifier

I had some amazing experiences with high end Woo Audio tube headphone amps.

I have a pair of Seinhauser HD 600 which are my go to headphones, and have owned the HD 660's, and love them both balanced, with my Hybrid tubed Schiit Mjolnir 2 headphone amp. I have been having fun with tube rolling with this amp. This Headphone Amp/Preamp amplifier has been discontinued but you may find one used at a reasonable price. I see that they now have the Mjolnir 3, which is solid state, but I don't have any experience with this new offering. The size of this unit may also be an issue (it's quite large!). 

@soix + 1 - and even if the built-in one is not 'really bad', it would likely still be noticeably inferior to a relatively inexpensive but good separate headphone amp, so I would suggest looking into one.