how far could I push my HD600 to subjective perfection?This depends wholly on what “perfection” means to you, so what areas would you like to improve upon over what you’re hearing now? This will largely influence what amp to recommend — and DAC as well because that’ll need to upgraded too.
I’d actually suggest going to listen to some better headphones and amps/DACs out there (>$1000) to maybe help you clarify what “perfection” really is for you. You may decide it’s better to upgrade to better headphones rather than to seek perfection with the HD600s. Just another possible way to go.