Sounds like a sawtooth (probably on multiple octaves), with an enveloped filter. The filter's envelope has a fast attack, and a long decay at a low level, like so:
Much like a funk synth bass.
This video explains how to do it:
youtube . com/watch?v=RJMx3kUwM34 (sorry, can't post links yet)
Just make the envelope not turn the cutoff completely down. You don't need Massive like in the video, any softsynth with a saw wave and an enveloped filter can do this.