Got 'em!
SRH1540 barely fits my head at full extension. Very awkward to fit. Not so sure about this bass.
SRH1840 is easier to fit and is more comfortable. Sounds much, much better so far. Really enjoying the sound.
EDIT:
Did some more testing but started to pads swap. OMG! It's the pads that create their different sounds!
SRH1540 with velours sound SO much better than with alcantara. Sounds almost like the SRH1840 stock. Although, they are even harder to fit my head.
SRH1840 with alcantara sounds pretty bad. Bass bloat galore. Sounds almost like the SRH1540 stock.
Going to compare both with velours/alcantara side-by-side when I get home from work.