I had the blackface 501 for years and loved it. It has very little color to it so I can use it on vocals, guitars, bass, pretty much anything.

I bought one of the ones with the push-in switch so I could have another one but it sounded awful. Got rid of it. It had the dbx/THAT VCA in it. Luckily was able to find a couple more of the good ones with the Allison/Valley VCA.

Regarding the problem of different levels with two different units. I suspect that one has the transformer option. If you are using the 1/4" outs, they are electronically balanced so a non-TRS connector will only give you have the signal. Hope that helps.

Using the two different versions in a stereo link? No, the circuits are too different between the two to get reliable stereo operation.