I'm not sure exactly what you mean by 'isolation'. If you mean like in a booth with me in a control room, then no. I just had the amps about three feet away from me, with me sitting on the floor in front of them.
If this is the problem, why not with higher-output pickups? Why does reducing brightness reduce it? You'd think clipping is clipping, and since low frequencies require more energy to cleanly reproduce, they'd clip first?
Isolation means in a booth with you in the control room. When the amp is in the room with you you don't hear the pick transient distorting for a variety of reasons, the most important being that you are not listening the same way a mic hears the speaker. You are typically off-axis and not two inches away from the cone.
High output pickups have less high frequency response. Clipping is amplitude dependent. A guitar amp boosts high frequencies therefore those clip first.