funny you should say this as took off my sprenger stirrups and replaced with lightweight wide tred stirrups for about a year......I have gradually been getting bad knees over every "more demanding ride" such as hunting/fun rides.
I put the sprengers back on the other day and bang......instant relief from knee pain! So I highly rate the sprengers (although mine arent bow balance) and dont know why I tooked them off in the first place!
I also have bad knees. In fact I used to not be able to trot for more than a few yards -oddly, though, galloping was never a problem
I bought the Sprenger bow balance stirrups - and I have to say that they are totally amazing. Practically no knee pain at all while riding - and none at all the next day (when I used to spend Monday and Tuesday hobbling around the office after a couple of weekend rides)