Turbo chicken,
I don't think you can actually use the EQ to make your bass sound fatter.
Actually, if you are looking for the commercial "Fat" sound, there is a couple of ways to do it.
1. Use the big bottoms. It is a APHEX hardware but has a software version (which only works with protools TDM systems.
2. Use the waves plug-in "Maxx Bass" it is a RTAS (protools LE) and VST plugin.
3. A technic I have used a few times before I have more expensive gear.
Send your bass out to 2 AUXes. Alternatively, if you are using software, you can duplicate it twice to get 3 bass tracks.
Keep the original.
To the send bass track or aux send, Shelf off all the highs. Then compress it so that it is really up front. play it along with the original bass and mix it in slowly until you get a thick bass sound.
To the third track, shelf off the lows at about 100 HZ and then chorus like crazy. Also mix this back in moderation to taste.
When all the bass tracks are played togther, you will get a thick milk shake!!
As with everything, don't overdo this or you will get too much low end saturation resulting in something that sounds like bass overdrive.
Hope this helps
Robin