Preamps

But since the subs are self powered I don't see how the preamp output impedance can affect the sub.

The AR preamp with output coupling cap, has to drive not only your main amp input impedance, but also the 10kohm of your subs amp even though there are 2 outputs, and this could be the problem, the output cap in the AR  "may" not be big enough as explained in my last post, and cause a premature rolloff of the low frequencies, so the sub may not go down low enough.

Cheers George 

Vac

VAC is transformer coupled. 

ARC has a fact sheet on their website expressing concern about the tube preamps driving a main amp plus sub. it’s in black and white unless they pulled it. it killed a ref 6 deal for me.,

just looked at arc site. article called the thorny issue of impedance. it’s located in a ‘tips’ section.