Massive diffrence between the sound at home (monitors, headphones etc.) and the live and loud PA sound

But why waste time moving furniture and stuff out of the way in order to place monitors in the center of the room, where you get the worst nulls, just to learn? I mean, if you have the time and patience then go right ahead. Personally, I wouldn't bother :-).

Ah, good question... I guess it's just because I like to experiment, and daily learn a lot by just trying things.
Some people don't have a clue about how just positioning cabs in a room may affect the sound. I've found that learning and experiencing things enlarges your culture, your ability to solve issues, your creativity.
It's like learning how to play fast: why should I practise at a fast tempo if I always play slow? For example because cleaning the way you play fast will also improve the way you play slow.

What's the use of putting a CD player on a marble floor and then on an old, floppy wooded desk, and comparing how this affects the sound of say an old, distorted recording (for example Callas)? None, if you don't care. But I've learned a lot more through this fast experiment than in 5 years of Engineering studies.

I'd agree tho that if one is not interested in the matter it's almost wasted time.

Hey, no problem! But correctly placing your cabs is critical anyway, and you'll have to experiment unless you pay someone to acoustically measure your room and provide a professional treatment.

What can be generically said is that each material (absorbing or damping) and shape of surface is designed to treat some frequency ranges. For example, you can't expect eggs cardboard (?) to be effective in treating wave lengths far from double their peaks-valleys distance.

At the same time you'll need different shapes and materials for each range of frequencies, so you'll need to realize in first place what are the offending ones.
Usually there are many problems where 2 (or, worse, three) walls meet: those areas need to be treated. Also, the more cubic the room, the stronger the resonances. Not all the rooms can be completely "cleaned".
The ideal room would be one with three different sizes (reciprocal, prime numbers) and no parallel walls\surfaces. The less the "real" room resembles this scheme, the stronger resonances will be.
And this is apart from the reflective\absorbing nature of the surfaces, which has to be taken into consideration too.

If you have time, have a sine 20-20kHz swap play in the room several times, and change your position, specially staying by corners. You'll see what happens in your room.

HTH

If the OP wants to learn about this stuff, the GIK Acoustics website has lots of valuable information on the subject in their EDUCATION section. They also have videos about how their products compare to foam, which is NOT recommended. He can also download this free Room Acoustic software, called Room EQ Wizard, that's really good for testing. Keep in mind that testing a room takes a lot of time since, in order to get an accurate reading, you need at least 10 samples of each position in the room as well as a very transparent omnidirectional microphone. That will give him more valuable information than simply playing a 20Hz-20KHz sine wave, especially if he doesn't know what to listen for.

http://www.roomeqwizard.com

Again, the GIK Acoustic guys are one of the best in the industry and provide innovative products that cover any aspect of room acoustics at a reasonable price. And they provide FREE room acoustics advice that's specific to your room. I mean, you can't beat that! And you'll be saving tons of time because learning about room acoustics is something that takes years to learn properly. These guys are pros and will give you the advice you need.

I don't wanna sound like a broken record, so I'll leave it at that. I've already given what I think is the best advice I could on the subject :-).

Take care!