Ah OK when I looked up the Goliath on SWR's site it said 400 W. Maybe that was a different year or model.
The level knobs on the power amp do not control the wattage. They just reduce the level of your input signal by a fixed amount before it feeds the power section. If you use the knob to turn down that level by say -12 dB, and then play a spike that's +12 dB higher than average, it's as though you didn't turn the knob down at all. So the power amp is capable of putting out its full rated wattage at any setting of the level knobs (other than fully "off").
It is feasible to use your existing cabs at 8 ohms each, with 700 W into each, one cab per channel. You have to be careful about blowing the speakers, but it's not unreasonable. Be aware that the single 15, in addition to being rated for only 400 W, has much less surface area than the 410, and will fart out MUCH sooner than the 410. So using that cab will limit you pretty significantly--you'll be carrying a 3800 W power amp and getting only 400 W of use out of it (roughly, based on 200 W per cab before the 115 farts out).
It actually would be a bit safer running both cabs in parallel (for a 4 ohm total load) off just one channel of the amp. Each channel is 1150 W at 4 ohms, so that way each cab would see a maximum of 575 W, instead of 700 W. You'd still be limited by the performance of the 115, but with a bit less risk.
