5 Amps at 120 Volts is 600 Watts. The only problem is the APC may draw up to 12 Amps when re-charging its batteries after an outage. I've seen that happen at work. Plug two UPSes into one breaker, all the equipment behind them presents less than 15 Amps of load, but after an outage both UPSes end up pulling 12 Amps to recharge and the breaker trips.
But as long as you're not playing at max volume right after the power comes back on, you should have enough power to spare.
Regarding the plugging of high draw devices into a power conditioner. It's not the conditioner which breaks. After a large inrush, for some reason they often over volt into the device, and either trip it's protection (blow the fuses in a sub or amp), or take out their power supply. I don't recommend you try it unless you know for a fact that the conditioner is well designed to handle high draw.
