Take a look at the back of your amp and you'll see a little message about this under the speaker jacks.
Why does this work? Hmm. I consider myself a pretty smart guy, but this particular item just messes with my head. My amp tech explained it to me, in his typical mad-scientist fashion, and I nodded the whole time, going, "Uh huh, uh huh," with my face all screwed up, and walked out of his shop with a razor-thin grasp on what he told me... which evaporated by the time I'd loaded my amp into the car. So I'll explain as best I can, but I am 100% positive that an electrical engineer, or anyone else here who understands this better than I do, will spot howlers all over it. But it goes something like this.
The output impedance ratings as labeled on the amp are accurate when the amp's running at full power (100 watts). When you cut the output power in half, the output at each speaker jack will also be affected; but it fiendishly means the output wants MORE resistance from the speaker than it does normally at full power. The speaker's higher resistance demands less current from the amp to drive it, which prevents the output transformer from working too hard to push the load; it's an appropriate mismatch that keeps things in equilibrium.
Effectively, it means you'd also be right to match a 16 ohm load with the 8 ohm output in this situation. I thought of it as meaning that LESS resistance in this situation would require the amp to reach farther to meet the resistance properly; the speaker's impedance provides the output with something to push against. If it doesn't get enough resistance, it has to gather more resources to push the speaker properly.
The manual illustrates a bunch of safe mismatches, but I think it avoids this whole question because a) leaving things as-is also constitutes a safe mismatch and b) this gets complicated when you're talking about an amp that can footswitch between channels running at different output settings. I think they just figured it wasn't worth making a big deal out of it because it might look like a design flaw... but I don't hold it against them because I think engineering an amp that would also switch output impedance settings when you switched channels is probably more complicated (and expensive) than it sounds, or is worth. It's also not a sexy thing for the marketing department (
"Now with output impedance switching!"). It also is further complicated in amps that have the 10 watt option... which will undoubtedly sound 1000% better if it's hooked up to the proper load, which I don't think it is under stock circumstances.
Dr. Science has spoken. I could be wrong about that last bit -- maybe Mesa did actually figure out a way to noodle with output impedance matching for the 10 watt setting... but I doubt it, mostly because of that little note on the back of the amp.
