Jack,
Let me rewire the one in the picture so it has the necessary mixing resistors. Email your address to me and I'll send it off to you as soon as I get it modified. Try it out and let me know how it works. I don't have a stereo jack on my Series bass, so I can't actually try it.
If it works out, I can make more for anyone else that is interested for $20 (s/h incl).
Thanks,
Loch