A year ago, I sold my heavy gear and changed to a Genz-Benz Shuttle 6.0 head with a pair of their 12T speakers. Throughout the year, I didn't find any practice or gig where I needed the second speaker.
Last night we played a gig in the country; the setting was an equestrian barn about 50 feet wide x 200 feet long. I brought both speakers, eager to finally play in a setting where the second speaker would be useful. The guitarists play through 60-80 watt amps, and there's a 1000-watt PA.
As it turned out, I could easily have played the gig with a single 12T, but figuring that double the speaker area would be good, I used both. And the total volume required was - less than 1/3 of the master volume. I had the preamp gain at about 75%, and the amp sounded wonderful. Everyone in the band was impressed with its sound and power. The only challenge was keeping it down so it wouldn't dominate the whole band.
When I read comments on TB about guys who say they need more power and headroom so they upgraded to the Shuttle 9.0, I cannot figure out what they're playing and where. They must be playing at ear-bleeding levels....or in NFL stadiums. One 12T speaker connected to the amp yields 375W, two speakers drops the impedence to 4 ohms and unleashes 600W. That's a lot of power!
The sweetest part is that the amp and one speaker weigh 36 pounds - and bringing the second speaker adds another 32-pound handful. My back likes me much more now!