they first thing i would recommend is to do some more reading. there is alot of info on this forum that you will answer your questions already. also alot of articles have been written that will really help yah out.
For us to understand more of what your hearing, you have to explain what the thin sound sounds like. There could be hundreds of reasons why it sounds thin. There is alot more to do with it then keeping the levels as close to 0 as possible without clipping.
My first theory based on what you have already said, is that you are overdoing it. One rule of thumb in recording, less is always more. Doubling everything will definately make it sound thinner. When you do a direct copy of everything like that, generally this will cause phasing in the signal, which usually makes things even disappear. Also, another big problem is that youve thickened everything up. You not leaving any space for things to come through. Only one or two things should be thickened, and ONLY if it NEEDS it. If it sounds thin in the first place, it has more to do with your tracking and the recording process than the effects and mixing.
If you for any reason feel you need to thicken something up, do it instead with a delay effect on the sends. Give the unit about a 5.4 millisecond delay and pan it a little bit off.
Reverb could also be a potentual problem with this. Reverb usually makes things sound farther away, more distant than non. If your going to use any reverb when you want it in your face, use it very sparingly, just enough to glue the mix together, and only put it on certain things.
Use effects sparingly, not even though pros just throw effects on their. Some do but they know how
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other than that, do some more reading around here.
danny