I wonder whether there would be much of a market for a device as you describe Roger, especially given the inconvenience of having to disconnect speakers to use a set of cans.
Many amp manufacturers are now starting to re-introduce headphone jacks into their preamps, integrated amps and even power amps again. And as you know, a huge number of vendors have popped up in the past 10 years selling dedicated headphone amplifiers. Less of an under-served market than before.
My thinking is that someone who is serious about listening to headphones, will more likely choose another, more convenient option amongst the plethora available rather than this type of kludge (e.g. a limited audience).
I would suggest instead focusing on introducing a preamplifier with a headphone jack, phono stage (so many people have come back to vinyl now), and really turn peoples heads on end by including bass and treble controls - in one package. Deja vu all over again ...
If you design a dedicated headphone amp, it will be one of more than perhaps hundreds out there now (take a look at the head-fi.org site - the DNA Sonnett is the dedicated tube headphone amp "du jour". The drawback is creating another me too product where there are so many now.
Looking holistically at Music Reference, I really think you need a good preamp to mate with the RM-10 and RM-200 products. You know how to design a great preamp (RM-5) Roger, why not instead introduce (or re-introduce) a preamp with an outstanding headphone jack.
However, the ultimate for me would be a Music Reference integrated amplifier along the lines of the new Luxman SQ-N100 and Leben CS-100x. Harks back to the good ol' days of the '70s before hi-fi went nuts.
Headphone jack, bass and treble controls, 6BQ5 output tubes, a phono stage, all in one neat package. You already have a great starting point with the RM10 and RM5 to integrate. As the boomer generation retires, I seem many of them going back to simpler stuff (the beauty of an all-in-one) vs. the hassles and the space consumption of separate components. I myself have been pondering a move to perhaps this cute little Luxman all-in-one ... it uses my all-time favorite output tube, includes a headphone jack, bass and treble controls, and will create a lot more shelf space in my downsized home. Recent review by Six Moons says the Luxman has an outstanding headphone jack too. More dedicated components and more cables? Blech ... aim for simplicity.
