Regarding the buffer stage possibility, you might want to contact a gentleman named Tom Tutay, of Transition Audio Design in Florida. I don’t think he has a website, but his contact info can probably be found via a Google search.
I recall that he has custom made active buffer stages for some others here at modest cost (hundreds of dollars, not thousands), which provided excellent results in high quality systems. Although those cases involved impedance issues resulting from some preamps (including ARC models) having to drive power amps and subwoofers simultaneously. So I don’t know whether or not the output impedances of the buffer stages he can supply would be low enough to be suitable for use with headphones, even high impedance models such as your Sennheisers.
Also, keep in mind that the output stage of the Ref 6 would be loaded by the parallel combination of the input impedances of the buffer stage and your active speakers, which will be significantly lower than the input impedance of either of them. And it will also be loaded by the sum of the capacitances of the cables to both destinations. So depending on the particular numbers that are involved connection of a buffer stage could impact the sonics you hear from your speakers.
Regarding the transformer possibility, first, headphones that were used with older radios typically had much higher impedances than modern audiophile-oriented headphones, e.g. 2000 ohms, and also required considerably higher voltages. Also, if the turns ratio of a transformer is chosen such that it would step up the impedance of the phones to a value that is a suitable load for the preamp, in combination with the input impedance of the active speakers, it may step down the output voltage provided by the preamp to too low a level. I see that the Sennheiser requires 1 volt to produce an SPL of 103 db, for example, which is relatively low sensitivity for a modern pair of headphones. And if the Ether Flow C has a much lower impedance while having greater sensitivity, like many other headphones, the higher turns ratio (and correspondingly greater voltage step-down) that would be needed to provide impedance compatibility between the preamp and the phones would probably leave you in the same boat, despite the greater sensitivity and the need for less voltage.
And all of this is not to mention that the bandwidth and sonic quality of a transformer intended for use with older radios would probably be severely deficient for use in a modern audiophile-oriented system.
As I said, it would probably be simpler and better to just buy a headphone amp.
Regards,
-- Al
