Probably it works like 3 of Little Labs IPB in one box but suited for guitar level signal - to run 3 amps together without phase cancelation.
Has anybody knows more about this? Is it difficult to DIY?
Research is done. Case is solved.
Actually this grey box is called Phase Wizard.
This is custom built by Gentry Densley (Eagle Twins guitarist, they also have cab line called Hex Cabs and trying to do something about amps too). Phase Wizard consists of input and transformer isolated outputs with phase controls and ground switch as well as two switchable gain settings for each out. Also DI out is here. Running power is standard 9 VDC.
I think it is must-have tool if you worshiping two or more amps together. Yes, there are similarities with Little Labs PCP, but Phase Wizard is more than twice cheaper, sounds better (at least to my ear) and two switchable gains per out is great idea.
It is easy to contact Gentry on his Facebook page.
" - smile_man
