One thing that always catches my attention is where the volume knob is set. That's kind of stupid - it's a simple voltage divider and a couple of one cent resistors could set the divider to any level I wish. But I always notice whether the designer set the system so that full volume is just under or over clipping, as opposed to systems that enter clipping where the volume knobs are set at half of their movement scale. Just a little thing, but I hate it when you are into clipping before the knob has gone halfway through its movement.