Also, got a new SLA battery (12v, 12 Ah), which means I'm not running through AA like mad. However, the IRF510 in my amp is running really hot. Maybe it is because I'm pumping in square waves rather than the sinusoidal I think this was originally designed for, maybe I just have other biases off. Never did get the 5 watts I expected, so very likely I wired something wrong in this.
In any case, it has started thinking about maybe looking into class E amplifiers here (since I'm starting with a square wave anyway).