First off. Getting a cab or amp stand to just SLANT the cabinet from the floor upwards will not - and has never ever - resolved the problem. It's still not angled phase right. It's all about the actual beam. The thing to use is just ONE 1x12 sealed cab on a heft stand that's height adjustable and make the core center of the speaker point in an exact 90 degree angle to YOUR (i e guitarist) EAR. Not head, or just about the height of a guitar player. Right into the ear! No stack in the world will get you there. The 4x12 stacks, stacks up at the wrong height and give you PHASE CANCELLATIONS due to the many speakers involved and contributes to anyone standing near "not hearing themselves".
See, even EGNATER has these TONEBRAs* where it's a circle of spokes that you put in front of the speaker center to spread out the dangerous high end that has this "death ray area" right in front of it. That's why everybody else hears the guitarist but himself. But that one is just made to spare other people from what guitarist should hear anyway.
We have solved the following in our rehearsal space. We raised every guitar speaker, and including bass, PA monitors on hefty adjustable shelves around the walls, and make sure that every CENTER of the speaker is aimed at a perfect 90 degree to each players ears (they're of course different height), and NO TILT AT ALL. We lowered all volume by 6 db which is half and everyone hears each other now, and upper end too.
So, if anyone's too loud the one that is struck by this first and foremost is that player himself. Not once has anyone asked anyone else to turn down. Or UP mind you! Now, if this was possible in a live situation too I'd be all game. But there all problems turns up.
I have solved this with a Quiklok triangle stand which can take 25 kilo. A bass cab on top. However, since it is 120 cm and I am 5"6 there's a little more to go. On stage, the only real estate space you have left is always UPWARDS. I have the cab and a head UNDERNEATH it, so the 1x12 bass speaker points right 90 degree angle into my ear. Even if I turn on it it's still there, or walk away a few feet, it's stille 90 degrees pointing at my ear. The space underneath it I can use as a bass guitar stand. And has modified it to do so. However, as it is a bass, there's a 15" cab too, that MUST reside on the floor, which is separated from this. So this is my monitoring and the floor 15 inch cab is for the rest of the people in band and audience. Never, ever heard on single person complain to me, about either being too loud or being to soft. Soundmen engineers loves this. However, there's a caveat. If you sing vocals too, there's a pretty good chance that the voice mike will pick up the sound from the speaker more, that if it was on the floor, or away from the mike. However, bass and voice doesn't interfere with each others so it's much better to EQ out the bass at the mixing desk, leaking into the mike. On guitar though, they may share at least the same space in frequency, but then again, then you don't need to mike up the guitar amp!!!!
I'd wish the industry should make HIGH ADJUSTABLE amp cab stands, WITH the possibility to use as a guitar/bass stand underneath it all. Saves a lot of stage space! And for posers, yes, your brand and guitars would still be visible to the audience.
*similar to Deeflex that was mentioned earlier. It is all about the BEAM! 
