Upgrade - Front End OR Back ???

I feel your gear it top notch at this point - "bang for the buck" gains will be vibration isolation, power conditioning and cables unless you are already "arrived" with those.

I don't know what you're running right now but I can recommend from my experience. On the vibration side I'd recommend a SRA VR3 for the headphone amp and on the PLC side RSA Haley would be quite and upgrade. What cables are you running with?

Update:
recently purchased Esoteric N-01 and wanted to upgrade the X-03SE but stay in the Esoteric line (no other manufacturers lines are in consideration), ANY suggestions?
thanks much