By your description of "sludgy" I'm going to base my recommendation on tones from Crowbar and Acid Bath, and how I get my sludgyness.
Big Muff (I like the Green Russian variants)
Compressor, optical type
Bump up your lows.
Decrease you highs.
Use a pick and scrape the strings with the entire side of the pick to pluck the, or use the side of your finger to pluck strings.
Then put it through a squashy optical compressor.
Just give it a touch of compression
Complex set up.
EQ (I use the Boss GEB-7. There are better ones)
Big Muff (Again, I like the models based on the Green Russian)
Compressor (I think use a Dynacomp clone in this application, I'm not sure because someone else built it.)
Limiter (I use a Carl Martin)
EQ set to bump the lows just a little bit and cut the highs. I don't usually mess with the mids. The reason for a graphic eq is that it gives you more control than just what's on your bass.
Run it through the muff. tone control and sustain on the muff to your taste. Put the volume high enough to make your limiter stay on.
Set your compressor so that you have a little bit of the squash-and-bounce effect to it. That's where you can actually hear the compressor lowering the volume of its input and then coming back up to volume.
Once all that's done, put on your limiter
Set it how you like, just make sure you are always going past the threshold level so that you have one level volume output from the limiter.
Between the EQ, distortion, and insane amounts of comp/limiting, it gets pretty sludgy.
Then, you can add stuff like flange or chorus and it comes out really f()ck1n' metal.
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