#2
it depends on what you are going for, OD is just lower gain distortion, so if you need more gain you get a distortion pedal. I run a metal muff in front of my tube amp and it sounds badass.
make Industrial and/or experimental electronic music? Join my group!

Last.fm
#4
Just preference. Today I was just messing around with my pedals and I put my MT-2(usually I use this to run into the computer for nightly practicing) in front of my TS-9, the TS-9 actually made the MT-2 sound thick lol.
2004 Fender American HSS Strat
Hughes And Kettner Switchblade
Schecter C-1 Hellraiser FR
Epi LP Classic
Epi G-400
Yamaha acoustic
Boss DD-3 Delay
B.Y.O.C. MXR Distortion+
Vox Wah
Ibanez TS9 Tube Screamer