I've gotten a lot of my gear here:
http://www.unionlabel.com/carhartt-ub01-double-knee-heavy-duck-work-dungare01.html
They no longer carry the Union-made, USA-madeCarhartt double knee, but a couple of pairs last me 2 years each of hard wear. Guy from this site told me that Carhartt was still going to make some stuff in the US, but it was going to be difficult for a store buyer to "specify" USA-made given their new distribution system. Whether this is correct or not, I dunno.
I got one pair of Blackadder. Lighterweight. Touch as hell. And the Bantam weight are great during the summer. I'd like to get the optional knee pads. That'd save wear and tear on the body.
I've seen the Duluth but never bought. Seem tough as nails from the ad and what you guys are saying.
Filson -- I like their gear (though they've gone "import" on some clothes), but too expensive for dirty work (for me anyway).'
One problem is that most skimp on quality fabric. Even Carhartt nowadays. All the good shuttle looms that made the old quality fabric at a slow pace was sold off (a lot of it to the Japanese).
Anyway, like my boots, my work trousers are something I invest in. I like to buy my gear every couple years and then not have to think about it.