There are 9v Batteries and then there are 9v Batteries... which one is yours?

If you take the typical PP3 style just as an example (aka 0068, 6F22, 6LF22, 6LR61, A1604 etc etc) then at one extreme it comes as a Leclanche (zinc-carbon) a real cheapie, with capacities starting from as little as 150mAH - and that's from some big brand names!!!! (those should power your equipment for about 10 minutes before the terminal voltage starts seriously collapsing - assuming their internal Resistance is low enough to suck 300mA out of them in the first place), right up to Lithium Cells at the other extreme that have 1500mAH Capacity, cost 30 times the price (and should power your equiment for around 4 hours).

Rechargeables (in that style) don't give 9v and therefore I haven't mentioned them.

Wall cube suddenly starts looking like a nice option...