What style/genre are you playing? "Fat" can mean different things to different people.
try lowering the Definition parameter and maybe use a Green Scream or One DS with Tone at noon or lower. Lower Presence.
adjusting Clean Sense/Distortion Sense has no impact in the tone btw.
hth
Don't be afraid to add some solid reverb as well. That can help add depth. You'd be surprised how much reverb is actually on guitar tracks. Go to YouTube and search for "Panama Guitar Track". You'll hear Eddie Van Halen's studio guitar track for that song and it sounds VERY different when it's isolated from the rest of the band. You can hear how much reverb he adds into his songs to help fill it up.
And in the amp section: increase power sag.
Try lowering the master volume, and raising the volume on you JBLs. I'm not really a high gain guy, but when I'm playing a profile with too much bite, backing off the master volume and raising my amplifier volume does it for me.
Also, If you use a volume pedal, go into the rig menu and see what position you have your volume pedal in. Pre or post can have a big effect on the gain/tone when using a volume pedal.
If none of that works. Press the Cab button and start scrolling through different cabs. You might find one that you like better for a particular profile.
try lowering the Definition parameter and maybe use a Green Scream or One DS with Tone at noon or lower. Lower Presence.
This is a big one. Sometimes I'll take a rig that started with Def around 9 and end up with it down at like 3, with the Green Scream or Treble Boost in front to clean things up. Also potentially helpful, I like to use the amp's Direct Mix parameter very gently, say 0.5 to 1. It helps retain some of the body from the guitar tone if it's disappearing in the amp.
Anyway, you mentioned Nirvana. I did a quick Google to see what Kurt used on Nevermind and came up with (among many other things) a Mesa Studio .22 and a Boss DS-1. Off I went to the Rig Exchange, where I found a fairly recent collection of profiles by 79 Sound. After a quick bit of tweaking...
https://dl.dropboxusercontent.…/teen%20spirit%20test.wav
Not amazing, but definitely in the right ball park. Definition at 2.2, Power Sag at 3, the amp's Compressor at 2.1, and then a One DS being kicked on halfway through. That's it.
This is a big one. Sometimes I'll take a rig that started with Def around 9 and end up with it down at like 3, with the Green Scream or Treble Boost in front to clean things up. Also potentially helpful, I like to use the amp's Direct Mix parameter very gently, say 0.5 to 1. It helps retain some of the body from the guitar tone if it's disappearing in the amp.
Anyway, you mentioned Nirvana. I did a quick Google to see what Kurt used on Nevermind and came up with (among many other things) a Mesa Studio .22 and a Boss DS-1. Off I went to the Rig Exchange, where I found a fairly recent collection of profiles by 79 Sound. After a quick bit of tweaking...
https://dl.dropboxusercontent.…/teen%20spirit%20test.wav
Not amazing, but definitely in the right ball park. Definition at 2.2, Power Sag at 3, the amp's Compressor at 2.1, and then a One DS being kicked on halfway through. That's it.
The attack remains a bit biting.
I think it happens at the low frequencies.
But when i plug my Kemper in rehearsal in a soundcraft mixer, with QSC amp and 2 JBL JRX 215, all the efforts i had made to obtain this fat sound went in smoke.
After looking at the JBL JRX 215's spec sheet, there seems to be a dip a bit above 400Hz (and predictable repeated at around 800Hz), influencing the area from about 350Hz to 550Hz. Since these graphs are always smoothed to some degree, the dip is probably even more severe than pictured here: [Blocked Image: http://i40.photobucket.com/albums/e229/donelectro/Vollbildaufzeichnung%2020.11.2016%20130147-001_zpsvpm36nav.jpg]
Have you tried to simply turn up the Mids in the Output menu?
I don't think around 400 hz would give what most people would think of as 'bite'. Low mid hz would add more warmth than bite imho. Maybe it isn't the best word to use.
Looking at the graph of your speakers though, it might make sense to add a multi band eq in and add around those dips pointed out by Don.
Great that the clean sense setting has helped though! Good luck.
I think any problem with the JBLs are not in the 400 Hz dip - rather in the lifted highs. From 1k to 3k, and even more above this. Not too little fatness - just too much sizzle 
Yeah good point too. Have you tried cutting the highs at say 8k to see what happens? Also in an eq
I just tested the "clean sens" and it's effective to get the fat sound i wanted.
There's less biting, very less.
Clean Sense has no effect on the tone.
Could it be you were clipping the input of the JBLs?
After looking at the JBL JRX 215's spec sheet, there seems to be a dip a bit above 400Hz (and predictable repeated at around 800Hz), influencing the area from about 350Hz to 550Hz. Since these graphs are always smoothed to some degree, the dip is probably even more severe than pictured here: [Blocked Image: http://i40.photobucket.com/albums/e229/donelectro/Vollbildaufzeichnung%2020.11.2016%20130147-001_zpsvpm36nav.jpg]
Have you tried to simply turn up the Mids in the Output menu?
I don't think around 400 hz would give what most people would think of as 'bite'. Low mid hz would add more warmth than bite imho. Maybe it isn't the best word to use.
Yep. It should even be the other way round. Cutting here with a EQ should make the guitar sound less cardboardish and thin.
I think any problem with the JBLs are not in the 400 Hz dip - rather in the lifted highs. From 1k to 3k, and even more above this. Not too little fatness - just too much sizzle
This plot is logarithmic. The "loudest" freqs are > 5kHz. But yes, 1 kHz is also a bit loader. This could really be a thing for an EQ (in the output section).
Just one general note to those speaker frequency responses. They are plotting the ideal response, either without walls or more typical: in a completely non-reflective room (aka over-treated)
The real response in a small to medium and untreated room at the location of hearing is *way* different. Rooms can change some frequencies up to +/- 20dB. Less in the treble range, much in the mid range and very much in low mid to bass. Even worse: the particular frequencies which will be cut or bossted by the room is completely depending on the position of your ears in that room! With ears closer to the walls things tend to get worse.
To treat this with EQ is hence very tricky. It will work - to some degree - at the listening sweet spot of a stereo pair. ANd then just one listener at time will hear the correction as good. For the persons left or right it might even be worse than without EQ.
Despite all of that: there those fat guitar sounds out there and they seem to translate from car stereo to kitchen radio to hifi in the living room or headphones quite nicely! Thats because one of the causes is not EQ. Think about the difference of a single coil and a humbucker try to find what the essential difference might be... 
