My understanding is that turning off one of the amps stops the preamp from using that signal path.
However it does not stop the preamp from being loaded by the combined input impedance of both amps, given that almost all preamps providing dual outputs drive both outputs from the same output stage.
Consequently I would rule out the Rogue preamp that was suggested. The balanced input impedance of your Benchmark amp is 50K; the balanced input impedance of the Luxman amp is 34K. Assuming those numbers apply when each amp is powered down (which is not always the case) their combined impedance is (50 x 34)/(50 + 34) = 20K.
Stereophile measured the balanced output impedance of the Rogue RP-7 as rising from extremely low values at mid and high frequencies to a very high 5.64K at 20 Hz. Not a good match, given the wide variation of output impedance coupled with the ratio of amp input impedance to preamp output impedance of less than 4:1 at 20 Hz.
The Makua, Ayre, and W4S preamps that have been suggested should be ok in that respect. Not sure about the EAR.
Good luck. Regards,
-- Al