Thanks for that input, RHL. I would point out, though, that the MC-275 in particular, and also most and perhaps nearly all ARC power amps in particular, have considerably lower output impedances (corresponding to higher damping factors) than most other high quality tube amps. And although those numbers are not specified for the various VAC models, I would feel very confident that difference would apply in their case.
And in fact the MC-275 is arguably almost in solid state territory in that respect, with a specified damping factor of "greater than 22" for the latest version, corresponding to an output impedance on the 8 ohm tap of less than 8/22 = 0.36 ohms.
Although the ARC GSi75 integrated which Pokey reported to work beautifully with the Sabrina is something of an exception in that regard, among ARC amps, having a specified damping factor of "approximately 4," which corresponds to a higher output impedance than is typical for their products. But assuming the 4 ohm tap was being used when he listened to it, I suspect the corresponding output impedance was probably still lower than on the 4 or 8 ohm taps of most or all VAC products.
On the other hand, though, there can sometimes be a tradeoff between which tap is most suitable for a particular speaker, and which tap results in the output stage of a particular amplifier seeing a load that it is happiest with (e.g., which results in the least amplifier-generated distortion, or amplifier-generated distortion that is least offensive in terms of its spectral distribution). So, yes, for that reason among others it would be advisable for Rinpoche to try all of the available taps.
Regards,
-- Al
