[Otto]

Yeah, makes sense to have it be 6 dB down. I edited my post to include descriptions of the graphs. Note that REW has calculated two different responses for a fourth-order 40 Hz LPF.

I think it's a bug in REW. You can cause it to happen by playing with the "Cutoff" and what I'll call "slope" controls in REW. Change slope, change cutoff, change slope... Something like that. You'll see it change the x dB down point for the target.

Wonder if anyone (myself included) has taken it as the target, and later had problems integrating with mains...