Hi. I've not had direct personal experience of the variants, if indeed there are variants. It's a sad fact of audiophile life that some sellers, either with an over-active imagination, or perhaps to try and mislead through adding value, invent so called upgrades. I'm unclear if this is the case here though.
My suggestion would be to find the official Audio Research website and look for a vintage section and/or email them as to the product evolution of the SP11.
Incidentally, the SP11 is a two-box unit and what you might have thought was the power amp is in fact the power supply.
There is, as you can see, some 'homework' for you to do. Worth it though in the long run.
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