The 6600 didn't have enough power to do much of anything, it ate hydraulic pumps which if you have ever changed one of those expensive little buggers it's not something you want to do twice. Not enough weight in the butt of it to keep it on the ground all the time. AIR CLEANER IN THE GRAIN TANK dumb idea - especially with the pre cleaner sucking in corn every time it got full. Kept spitting out fishbacks off the walkers, broke the top sieves in the front, sheet metal started cracking in the back at 3100hrs, cooling system was too small, cab was off to the side, unloading auger too short and at too steep of an angle, electric clutch for the head didn't stay engaged all the time, could not get a clean wheat sample out of it no mater how hard you tried - would sometimes put the whole straw and everything in the tank, soybeans it would throw pods in the tank, it was the only combine that I ever ran that I could not adjust, so ya know, just a few things. On the up side it did run nice and the air worked great. Even if your 1420 doesn't keep up with it I would trade a 6600 for a 1420 any day of the week.
Bum deal on totaling out the 1640. If I could find a nice one with a cummins and those hours for that price I would jump for joy and pick it up tomorrow. I can't put an extension on because of the clearance going into my barn, that and none of my fields are over 1/4 mile long, as long as I can make a whole round I'm happy.