Mal,
I use the unmodified Vampires, though I like Dennis's idea with the CM1. However there isn't much room there to make a nick, or whatever, to allow soldering the ground to the body.
What I have done here is to wire one input direct to the attenuator. The other input goes thru the selector switch and then to the attenuator. Bypassing the selector switch, 2 pole Elma switch, seems to make a small improvement, but it is small enough to not matter all that much. I just seem to be in a real "minimalist" mode at the moment.
But in using the direct connect the selector switch does have to be switched to an unconnected lug to make it "dead".
I dunno, I could be all wrong here, but my direct connect seems to work for me. I guess you could try both ways and try to hear if there is a difference.
BTW, I pretty much only use one source. The switch and extra rca's are mostly for looks.