OP's post describes a warpig perfectly.
It will make your g/b/e strings very thick and smooth. It takes out the higher end spectrum, so instead of the glassy jangle, its just thick and preserved. Rounded off is a very good description.
Has good bass, lots of low mids, and lots of clarity, and will clean up. The bass is big, but not loose. I wouldnt call it tight either, but its tight enough. Sludgy is the last word i'd use to describe it.
Plus, if you just roll the volume back a tad, it will roll of bass and put you closer to vintage territory.
You want a warpig.
In response to other posts: the warpig is not very modern sounding. heavy as balls yes, but i wouldnt say "modern". With a 500k pot, it pulls of vintage gain very very well, especially with the volume rolled back a bit.
With a 1megK pot, it will add treble and give it more of a modern feel.
I should mention, by vintage gain, i mean you can hit the Black Sabbath Warpigs tone PERFECTLY.