When run from the batteries, the A/C line is literally disconnected from the battery /amp. It would therefore make no difference what kind of cord is used when you are in D/C mode (nor would the cord help or hurt the ability of the battery to charge -- any power cord would suffice for charging).
I agree that the Model 6's (and 8/9ti), when run from batteries, are supurb amps. I tend to avoid comparing well-made equipment, but am confident, based upon my friends' experience and my own experience, that the only current production solid-state amp that is clearly better than that vintage of Rowland amps is the darTZeel, and the few amps that are marginally better are all much more expensive (Gryphon, Boulder, Lamm and Tenor come to mind, which are either biased in Class A and will roast you out of the room, and/or use tubes in their input stages and therefore cheat).