Ahhh yes, the eternal quest for more volume. Here's my situation: I play in a very loud band, and I've noticed that my Fender BXR300R head clips at anything above 6 on the volume knob, even with the built in compressor on. I'm using an 8 ohm Peavey 410TX. Would an 8 ohm 15" extension cab in parallel give me more volume (thus allowing me to turn my head volume down and hopefully reduce clipping) or should I start looking for a more powerful head?
Yes, you'll get more volume. First you'll get a bit more power out of the head, secondly the increased cone area will increase the efficiency of the system. It may not be a significant enough increase to satisfy you, but it is always worth trying. See if you can borrow or rent a cab first before buying. Oh yeah, your Peavey 410TX really scoops out a lot of mids. You can disable the crossover easily to get the mids back and this will make the cabinet a lot louder.
I figured it out. I completely bypassed the crossover for the 10"s, allowing them to be full range. It dramatically improved the sound. I actually had to cut some of the mids on my head. The clipping light on my head doesn't even blink anymore; it used to be on almost constantly. It's almost as if a dam has broken on the power coming out of the head. Thanks for the idea.