As Alex implies, what you're asking is kinda the wrong question. The real issue is not the amp to cab relation, it's the relation of the power you have to the power you're gonna need to do the kind of gigs you're trying to do. The idea is to get more than enough power for your situation. Which means you don't use it all, which, as Jiggy suggested, is what headroom is. Once you have more than enough power for your playing situation, the amp-cab relation (e.g., 2:1, 1:1, 1:2) is not that important, as long as the cab has enough capacity to handle the proportion of the amp's power that you're using on a regular basis. For example, if your gig requires a 200 W amp, and you buy a 600 W amp, you're likely in good shape regardless of your cab's power handling (within reason!). All else being equal, you don't gain any advantage (assuming, again, that you have more power than you need and aren't using all of it) from using a 300 W cab as opposed to a 600 W cab; you don't gain an ounce of headroom that way, because the amp-cab ratio is irrelevant to amp headroom per se. It just doesn't matter that much.
There are other factors that go into chooisng a cab, such as efficiency and frequency response, but for simplicity's sake I'm ignoring these for the moment.
Believe it or not, there's a TON of stuff on this subject in this forum. Search on headroom or power handling.
thats right, i see you stalkers
