I'm a little alarmed when you say "until it starts clipping, then back it off a hair." That little light should theoretically never go on. Mine doesn't. That's just to indicate too hot a signal coming from your bass (or pedals). Entirely separate from that, there's a limiter slider switch on the top of the unit. I keep mine on, even though I rarely run the amp at high enough volume to need it. There's no light or other indicator that it's working. Just know that if you crank it up too loud the limiter is going to engage and stop you blowing the unit. This surely explains the "I keep turning it up but it doesn't get any louder" phenomenon. It's a sign that you may be pushing the amp too hard--stop it.
Furthermore, how you can be turning the volume knobs this high with only a 212 cab under it is beyond me. I would think the speaker cones would fly across the room before that amp clipped
but I guess not!
You need a second cab, no question. If you're turning up that loud, engaging the limiter on the amp and still wishing to turn it up further, it's definite. Most folks advocate getting identical cabs, so a second 8 ohm 212 is what I'd recommend.
This will automatically put the load on the amp to 4 ohms and unleash up to 500w of power. Double the speakers, double the watts--you should find what you're looking for by doing this.
Forget the watt ratings of speaker cabinets. They're next to useless. They don't need to match your amp, they don't need to exceed it, they don't need to combine with one another into a magical formula. All you need to know is, don't turn your amp up so loud that your speakers are getting damaged. The only way to tell this is to hear them. The end.
So mostly I think you have things exactly right.
