Ouch. My knees hurt just thinking about doing 50 rpm. Personally, I prefer staying well north of 85 rpm, even up into the low 100's. It's possible that a higher cadence will solve your knee problems entirely.
As for the bike itself, I'd consider lowering the saddle a little, but not moving it along the rails yet. By moving forward on the saddle you were effectively lowering the seat height. Dropping the saddle will also move the saddle slightly forward. If you like the lower cadences, a lower than average saddle usually works better.
For me, assuming the bike fit is not terrible, raising my cadence would have a much greater effect than changing the bike. Therefore, I'd hold off on making any drastic changes to the bike until I'm sure that my pedaling style is not the root of the problems.
Once again, I'm no expert, these are just my experiences.