Matching wattage btwn heads & cabs - how important is it?

[beans-on-toast]

I'm a believer in the complete opposite of this. A high power amp will be able to deliver clean power and volume to whatever cab you have. If it can't get loud enough for your application you need to add more drivers or cabs to the equation. Make sure to keep the impedance matched with the amp (more important than matching watts). With the available power these days it's getting harder to find a cab even rated at a power equal to the amps rating. As said above, use your ears more than your eyes and don't let too much volume make bad sounds. Based on your setup, I would say you need more speakers to get the volume you want...not necessarily a higher power rated speaker.

A 300W amp into a 100W cab is not going to fair well at high volume levels. Adding additional cabs with the impedance matched will help increase the volume up to the point where you are not hitting the limits of each speaker. Adding cabs will also increase the power handling capacity of the cabinets. So more speakers does increase the overall power rating of the cabs, like having a speaker with a higher power rating. We are saying similar things in different ways. Both approaches have their tradeoffs.

You need to have a starting point when trying to match an amp to a cabinet. Amp to cab RMS power levels of 1:2 is one reference that the industry uses.