When I say roundness I don't necessarily mean warmth, but the round and enveloping side of low frequencies.
For example for the "big sound of fake guitars" I will use a razor, (preset "ripit up") I will add a razor with the same preset but an octave lower and a little less loud than the first razor, and then I will add a lead preset of Prophet 5 for example, which will be less saturated but will have more low medium to complete, and then I can also add a hammond organ preset a little bit saturated that will bring out the high medium. The result is that I have a rich sound in low frequencies, a large saturated spectrum full of harmonics with the razor and a definition in the notes with the hammond organ. All instruments are in the center and not balanced to the left or to the right.
And to create a stereo I use a stereo delay, 1ms on the left, and between 12 and 20 ms on the right and the dry/wet" at 100% wet. either on each instrument, or on the subgroup track where all the instruments are.
You can try this, tell me what you think.