Is it possible to rocketjump from below the stairs up to QUAD at the nail ammo?

Do you mean rocket jump from the bottom of the stairs down near the water? Yes, that is possible. Even more impressively, it's posible to rocket jump from there to the rocket launcher, though I've only seen Milton do it.

I´ve tried and tried and tried but keep bangin my stupid head when I try, or get stuck in the stairs

Does he do it from standstill or does he jump once first and then fire the rocket?

I think to get to high rl from by the nails you'd have to either have built speed up from previous jumps, or use more than one rocket.

You can see Ganon doing something like that here: http://youtu.be/CC0g6aFDq_Q?t=3m52s .

I saw locktar do it first, during one of his duels when he was all dm2 I can do it maybe 1 out of 10 or so, it has a pretty slim room for error that's all. I don't think I ever aimed to make it to quad, though, only as a boost rj to get to big

You can do it from the ground. No jumping necessary. Jumping would probably just make it more difficult.

Stev plz...

RJ from nails to high RL? Jesus. I'll believe when I see it. Demo, please!

Not sure it's possible to make it the quad from bottom let alone high rl, though knowing Milton I can believe in him attempting something like that in a live match and succeeding but I would like to see the demo. Closest I come up with: http://www.sendspace.com/file/1y3k2k
edit: turns out bottom to quad is very much possible, so is the second one, but the second one is done in a funny way. http://www.sendspace.com/file/jqc23c

Nice demos Timon!

I noticed you stepped on the stairs, is it possible to do it without stepping on the stairs?

I think one of the latter mvd demos is done w/o stepping on it but I'm not sure. If not, then maybe you need a tad bit more of horisontal velocity to not bang your head and make it to the rl.