Cab watt ratings are continuous up to thermal failure . Music watts are transient. Transient high energy spikes cause extreme driver movement. Thats the danger to watch. A 500 watt head wil never be pumping 500 continuous watts and cause thermal damage to the sp coil, a 50watt head driven to extreme certainly will melt a coil. But a500 can cause over excursion of the cone. So its fine to use 1000w with a 400w speaker, but the potential for mechanical excursion damage is real and has to be managed with a knob and your ears.
I use a pf-800 with one or two pf-115he as needed without issue.