i tend to sample the sound i want to begin with. that is, ill dig, and dig, and dig, and eventually ill come accross snares, kicks, loops, etc. that sound good as is.....those are what i use for my kits. with very little tweaking.
now of course, there is always the part of picking the right sounds to fit with the beat, etc. and sometimes that warrents some tweaking, but for the most part, for me anyways, its more about choice of sample then it is "secret techniques of bit conversion, comp, eq, etc."(not saying i dont use bit conv, comp, eq, just saying i dont have some secret technique)
the other option is as you mentioned, is to sample something and tweak it..........but generally speaking, if it aint grimey, hard, snappy, ill, or whatever to begin with, it might never be.
there lots of ways to get those gritty drums, its up to you. and it depends on your definition of gritty, grimey, ill or whatever.
in the end, its what sounds good to you and whatever it takes to get there.........but for me, sample choice is big part of it.
as far as tweaking techniques...i tend to find that layering,filtering, and eq gets me the results i desire more than converting bit rates. but thats what my ears tell me.
i definately like damu sound. there was a dude in the "your music" forum that had some ill beats taht sounded like damu, he might be a good person to ask about techinque.... this dude
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