Long story short: My buddy's 1960b is picking up audio and sending it back through to the recording interface. What the fuck.
DAW: Reaper (OSX)
Interface: Saffire Pro24
Rig1: AxeFX2, output1 runs into Saffire-Input3, output2 runs into a matrix/2x12 setup.
Rig2: MkIV + ENGL E570 routed through an RJM-RG16 with a G-Major. The Slave-Out from the MkIV is sent to the Saffire-Input4, this way we can record everything that hits the MkIV poweramp (his whole rig) and then slap a cab IR on it in the DAW and be done. Per the manual, the Slave-Out is designed to run through an external poweramp or something, but it works great for this application.
We've always had a bit of bleed from Rig1 onto Rig2 while recording (we record at the same time during practice and for demo tracks) but its low enough that in the mix it goes unnoticed and I haven't taken the time to mess with it until now.
I found out tonight that even with his rig completely turned off I'm still getting the bleed. Obviously if I unplug his cable from the Saffire the bleed goes away. However, if I unplug MY cable from the Saffire, it still bleeds. This led me to experiment, and I found that running a track through my XiTone (not involved in this recording process, we use it metronome & drum tracks) and hit it loud enough, I get the same bleed. So on a hunch, I pulled the speaker cable out of his cab and VOILA, the bleed goes away with the entire rest of the recording setup left alone.
So what the fuck is happening here and how do I make it stop?
I THINK what it is...the Slave-Out on the MkIV gets its signal direct from the poweramp signal. That is to say, whatever is pretty much the final point before the signal is actually amplified and sent to the speaker, a line is run from there to the SlaveOut. So I think because of that, the sound in the room is causing his speakers to vibrate and that slaveout tap is picking up those vibrations and sending them back through the interface to the DAW. ...not sure I can really do anything about that other than alter our recording process, but also not really sure how to record his signal direct without running it from that Slave-Out but still catching everything he plays through. Fuck.