I love the DOD FX25 but it has a VERY wide sweep and thins out considerably when the filter fully opens. I had to use a blender (later versions had a built in blend but it sucked) to not disappear in the mix.
I'd say to check out the Mr Black Fwonkbeta.
It wasn't my favorite filter but for downtuned neo soul stuff it's got a big, greasy envelope that would work well and not thin things out.
In general I'd be looking at low pass filters but a band pass filter with a blend (like the MXR) could also work depending on how it's set.