I use the Sennheiser HD650 with the Rx MK3a regularly. It is a good pairing in that the Rx has plenty of
power to drive these 'phones. I use the Middle gain setting and this has plenty of oomph -- so there
is power to spare. It easily enunciates the bass line and gives clear performance. Instruments are well
delineated from one another. So I don't think you'd be disappointed. Of course the Rx also allows balanced
operation if you plan to go that way in the future.
Since you don't need a portable, you really have a lot of options. If you are interested in vacuum tube
amplifiers, and are willing to assemble from a kit, then another choice to consider is the Bottlehead Crack
amplifier. As you'd expect this gives a more "liquid" sound and a bit more life in the sound of winds, strings,
and voices. The bass is not as tight on the Crack compared to the Rx... but you might or might care, depending
on your preferences. But the design of the Crack is very well suited to the 650 and gives a well controlled and
nuanced sound. I think not as much ultra separation between channels compared to the Rx.
This says more about my preferences than it does about the amplifier, but I very much like the Rx+HD650
combo for classical music. It helps pull apart the voices in the orchestra so that symphonic works come
through more clearly and with less muddiness. Solo instruments differentiate properly.
ALO also offers a portable vacuum tube amp. I've not heard this, but may be another choice to consider.