i don't know a stereo guitar amp except the Diezel VH4S…
the Voodoo Lab GCX is not a line mixer…it is a loop switcher…completely different things.
if you use a line mixer,you will always have your dry sound and the effected signal in parallel.
please,click articles and then guitar rack mixers:
http://tubefreak.com/tubefreak.html
^i am sure,this will help you in the right direction.
and look at those products:
http://www.rane.com/sm26s.html
Show More...http://www.rane.com/sm82s.html
http://www.rjmmusic.com/minilinemixer.php
http://www.masottiamp.com/mxm/
http://www.soundsculpture.com/products/switchblade.htm
Best,
Nikos