500 km straight without butt soreness? That's something! The damn thing is expensive, that's the only drawback. Which model do you have?
I have a 12$ one from a big european discount super market and performed beautifully in very long trips (German quality). I guess it's a matter of butt physiology and luck. But I'd definitely consider a Brooks after all that positive critique.
P.S.: Blood in stool? For real? I've never heard that. How is that possible?