We've done a few things to lighten our tandem such as wheels, CF seatposts (got rid of my thudbuster), my saddle and pedals were heavy too. It's nice to make these changes because we can tell when we pack the bike for traveling that we don't come so close to the weight limit when we have lighter parts, and it's also easier to lift into the van as well.
Anyway, the thudbuster must offer a much greater setback for the stoker than my current CF seatpost with 20mm setback. I still need to be back farther on the bike and we are out of options because the saddle is all the way to the end of the rails where they start to bend upward. I had no idea when I got rid of the thudbuster (haven't missed it and apparently don't need it for 95% of our rides) that I would struggle so much with getting my saddle back far enough again. Do any of you have ideas for other seatposts or ?? Right now I have a bit of knee pain under the kneecap and I'm thinking that it's because I'm still not far enough back. I also have shorter cranks than the captain (mine are 168) and I wonder if switching to a longer crank would help. I don't know what other things I might notice if I change my crank length, so I'm not sure it's a good idea.
I like some of the changes we've made, but I just had no idea such small adjustments could make a difference in how it feels!
One of the FSA K-force seatposts has 32mm of setback. Ritchey used to make a "wayback" seatpost with 45mm of setback. You might find one used somewhere. I think Nitto made a "wayback" aluminum post as well, might try the folks at Rivendell for that one.
If you changed to a seatpost with less setback you should need to raise your seatpost to get the same leg extension which could be the answer to your knee pain. generally a longer crank is harder on knees than a short one. Here is a good discourse on determining correct setback by a well respected builder. http://www.kirkframeworks.com/blog/2.../riding-tip-3/
Roubaix, Epic, Source 11, Stumpjumper, Cannondale Tandem
I believe the current model Thudbuster ST has a setback of about 15mm, although it looks like a lot more. Perhaps the problem is that you're too far back instead of too far forward. I think the original versions were straight with no setback.