Still working out the right value for the blend control, Moosapotamus suggested 500kB (http://www.moosapotamus.net/fsh1a.html (http://www.moosapotamus.net/fsh1a.html)), I tried 500k and 100k and it's still not quite right, the volume drops a reasonable amount in the middle of the sweep.
I think 50kB should do it...
looking at the sharkfin schem, take a wire from the board output and from the junction of c2 and r4, and run them to the two outer lugs of you blend pot. then take your board output from the wiper.
voila!
Edit. I've gone as low as 25kb which seems pretty optimum actually. Not getting a huge volume drop in the middle, and getting a good range of blend.
Extremely cool for bass!