I just got mine a month ago, but have run it twice in the field, powered by a Battery Geeks 5-12-50 Freedom E battery, and it's been fantastic. Though it is marketed as a battery for 12v DVD players, its output is technically 11v, but I was assured that it would run on my DR-680 and it does, just fine. I doubt there is much risk in powering it at 11v, but at 9v, I would be very skeptical. I very much trust the quality of these Batt Geeks batteries. It was $130, but I'd rather pay a little extra and know that it is a quality product, over buying some garbage no-name 9v battery on eBay for $30.
Edit to add: This could be a cheap way of defending the product, but technically it does say in the manual that the unit shouldn't be powered with anything other than 8x AA or the supplied AC plug. Like I said, cheap. BUT! It really isn't Tascam's fault that we're trying to do something that the manual says will cause the unit to malfunction. We take for granted that these DVD batteries power most of of our shit without any interference.