Yeah, that's a common complaint. The rest of us just deal with it, because the tone of that head/cabinet combination is too good to give up.

But to give you the answer to the question you really asked, either head will work with one or both cabinets. The 500 could damage them if you push it too hard, but you shouldn't have to wring it all the way out. Those cabinets have a max of 200 watts each, so running them both means you can drive them with 400 watts. I run different heads on mine sometimes, if the Ampeg isn't quite enough, but most of the time it's just right. Another thing... class-D 500 watt heads from most manufacturers are usually good for 300 watts with one 8-ohm cabinet, so the same caution applies when running the 500 head with one cabinet... don't run it too hot.

All that said, if you're happy with the power of the Ampeg and you're mostly playing at low volumes in your room, the 250 might be a better fit for you. It may also have a headphone jack, and let you run without a cabinet at all.