Sean's circuit works great. I made one recently for a buddy, who needed to mix an effect with clean, but also wanted to be able to compensate for overall level reduction created by notches. I gave the output stage a bit of gain (4x, I think), and stuck a 100k volume pot on the output. Normalizing the send and receive jacks, so that the one automatically feeds the other, if nothing is plugged in, turns it into a clean booster (though with modest boost). It has a 3PDT master bypass, which allows him to use it as an A/B/Y if he feels like it. How? Well, in bypass mode, you g straight from input to output. When engaged, if you only use the send jack, that can be your alternate output. Depending on how the output volume knob is set, that alternate output can co-occur with no signal from the "mix/blend" output jack (i.e., fully attenuated), or it can be a second output of equal or even greater level.
VERY flexible signal routing. Kudos to Sean.