fat = a strong fundamental frequencies
warm = a good amount one present low mid/bass frequencies 100- 300 Hz also detuned sometimes
defined = a clean sub bass and some higher mids to give the bass some presence
What you want is to take a bass and make it sound deep and warm. you can detune if you want, but what is more important -> make good use of the synths lowpass filter. Analogue filters have a warmer feel so you could try that. Then remove the lowest frequency with a high pass. automate the cut-off frequency to remove the fundamental on every note played.
Also try to saturate the bass to get some more frequencies into the bass sound -> more warmth
To the stereo thingy why sub bass should always stay mono:
We can't locate bass/we can't hear bass below c.a. 80 hz. We only feel those frequencies. So there is no use in panning the stuff around.
If you want a stereo bass then try to build one that is mono to at least 100-200 hz and then spread it. you need to layer some synths to do this in most cases or use modulation fx like choruses with a high pass.
Another reason why bass should be mono is because of mono compatibility. Stereo bass might collapse when you put it in mono like a lot of club PAs -> your track sounds weak.
But never forget the rule: what sounds right is right, but always check in mono!
Multiple voice bass is a good attempt is you use the different layers intelligently like 1 synth for sub bass, another sub bass one octave above then 1 saw bass patch filtert down and high passed then 1 subbbass that you distort 1 stereo detuned filter saw bass, one chorused/fxed sawbass one patch with a lot of transients, one white noise plug in and so on and so on. But really you make mixing and especially mastering a lot easier if you just use a mono sub sine wave since everything else might collapse or you can't hear it and i takes energy
There is no gain if you keep it mono. If it is stereo there might be a gain change if you mono it.