I have an Xp-50 which I picked up quite cheaply. many people don't enjoy the ROland setup, but as I have techno joy (as opposed to techno phobia...) I spend hours playing with things (take a distortion guitar out of the gm sounds, add pan delay, through a stereo speaker, after a bit of tweaking I had the heaviest darkest sound you could imagine). however I digress...
Taking the organ sounds I use aftertouch, effects, and re-combination of the patches to tweak the sounds. Effects first, patch tweaks where necessary...
One thing to remember for people like me who use organs predominantly as a backing sound, is that bass adds a lot to an organ, remember that most of these were originally played with two manuals (keyboards) and a pedal board, that's a big sonic dimension, and adds all sorts of spacial effects, bass caused distortion etc. etc. Personally, if I really wanted a great organ sound for a lead line, I'd get a dedicated organ module. Otherwise just be prepared to experiment, works for me...
Al
who loves his xp-50, all apart from that damn five octave keyboard!!!!!!!