The reality the listener runs into is that with the possible exception of the Apex Pinnacle, no pre amp section of a headphone amp can compare with a dedicated high-end pre amp. Obviously, pre-amps are designed to be used in conjunction with a power amp and since the HE-6 sounds best when plugged into the speaker outputs of a power amp my original statement stands.
Few speaker amps are equipped with a volume control. Those that are must have a pre-amp section which brings us back to square one.
The quality of sound reproduction will always be limited by the weakest link in the chain.
I like your taste in speakers.