Lisa,

I went from the B17S to the B17 so i could move the saddle back further. I had been feeling sort of like I was riding a unicycle. It helped a lot, but i eventually found that my sitbones were a bit too wide for the B17 anyway, they were too close to the side frame edges of the saddle, so I then switched to the B68 on both my bikes, which was as wide as they come. (I have very wide hips and sitbones to match) It's been heavenly ever since. i ride on the hoods and drops quite a bit and never have any problem with my B68 because of that. I am most comfortable when the saddle nose is tipped ever so slightly up from level.
I am so glad to hear that about the B68. Everything I've read (I know, you shouldn't believe everything you read) made me believe that the B68 is for upright "city" riding and not for longer distance drops/hoods riding.

personally found the B17S forced my whole body to be too far forward. The non-S with the longer rails, shoved all the way back, allowed me to get an inch and a half further back and I felt so much better balanced and off my hands.
I feel like my weight is too far forward too. If I end up with a B68 I will not get the S.

After I get the re-fit with the B17, I'll ride and see if there is significant improvement. If not, I'll probably order the B68.

BTW, I love your Rivendell. Amazing that it's the same seat post. Big difference with the 2 saddles.