So what happens when you decrease the load driven by the preamp below its minimum specification? The extra current demand on the output stage softens dynamic impact and results in generally poorer sound as the vacuum tube output stage struggles to drive the low impedance of the parallel load of the two connected amplifiers. Luckily, this won't damage anything. But sound quality is why you purchased Audio Research products in the first place, right? - and we never want to hurt the sound quality!
Faced with the desire to add a subwoofer or second amplifier, what can you do? Check the specifications page of the manual of the amplifier or subwoofer you are interested in adding. (Audio Research provides the input impedance of our amplifiers in our owners' manuals and on our website.) Use the equation above to see if the gods of the high end smile upon you or smite you.
Good luck finding the input impedance specs on the speaker manufacturers' websites! Of six well-known subwoofer manufacturers' websites I visited, only one bothered to specify its sub's input impedance.
Nobody promised this was a hobby for the faint of heart...