The audible difference from putting the resistors into the + path could have been completely imaginary. I figured it was simply due to symmetry; so the signal from my input buffers which gets split three ways all push into the same value resistors. I don't really know if this has any effect going into opamps though, so I'm taking that with a grain of salt.
My OL and OG circuits are basically differential circuits but with infinite Rg, at which point the + resistor can go to zero... I've tried it with the + resistor at 4k and Rg = 1M and I think it gets a little bit grainier; ideally I would use a bigger value of Rg or smaller + resistor but I'm working with what I've got. I think I read somewhere that having the Rg there helps correct for input bias, but then again if you have any sort of noise in the ground then that will probably show up...
I've got parts for a 3-channel a20 amp incoming right now. I need to order some more opamps too so I can actually use the same opamp for all three channels (right now I have a different one creating my OG, which I suspect will cause a bit of distortion due to slightly different response). I plan on testing the ABG circuit plugged into the a20 amp and fiddling with configurations and resistor values until I find the best sound, then I will finish building the b22. I might even go really crazy and build two b22's, one with the ABG circuit and one without (then sell one, because I've got bills to pay
).
I wish I could somehow test the outputs with RMAA, but I would need a dummy head and microphones... and that's a couple steps more than I'm willing to go for this venture which is really just a proof of concept more than anything.
The crazy idea with the M3 was to build it as some sort of modular unit that could be used independently (like a transportable unit), or for home dock it into the beefier b22 and and feed it the ABG signals (and power the M3 with the 4th output from the s22). Did I mention the crazy? Mwuhahah...