Gain button was off for me also. There was a bit of "hair" but overall the tone was that of a clean, powerful tube amp (and nowhere NEAR muddy!). If the gain was on, it would have been quite overdriven, and I definitely didn't want that.
That setting is very cool! I dig it. The Streamliner tone controls are so interactive. You wouldn't think that the tone you get from that setting would be 'what it is' with 'normal EQ logic'.
Your description is right on.... a super punchy all tube amp vibe, sounding like it is being driven to just the point of noticeable break-up. P Bass heaven there!
Of course, the absolute gain settings would vary a bit from player to player given technique and instrument output. With my moderately hot output Nordy P, and my relatively soft touch, that dual 3 o'clock setting on the initial gain structure hits that 'right at the edge of warmth and noticeable distortion' level.
Dig it!