By loud, I'm assuming you mean the overall output level, as opposed to preamp gain right? I like to have the speakers pumping, but not enough to start rattling the back of the cabinet, or making the speakers fold up etc. Enough to make a serious racket, but not enough to add farting noises etc. Detuning can add low end stress to a speaker cab, as most 4x12s weren't originally expected to have to push that amount of low end out, so if you're detuning (by that I mean down to C or lower) you might want to make sure there no serious rattling/rumbling caused by the extended low end. If I hear that I'll back the output gain down enough to clear it up, or maybe try adding another cab into the setup to accommodate the load.
And of course the amount of preamp gain will vary as well, but I don't think that's what you're asking so I'll stop here.
Neil K.
