Onkyo P-3000R gain adjustment?

I can't find anywhere in the documentation that says the gain is adjustable on this preamp. I've always just assumed that it's not. I've been very happy with the performance although the volume knob is always between about 9:00 and 10:30 in my small 9 x 12 listening room. Besides making the volume adjustment extremely sensitive, I've always wondered if I could adjust the gain so my listening is in roughly the 12:00 range give or take it might also improve the sound. I recently picked up a Violetric V281 headphone amp/preamp. From the factory default you can adjust both headphone out and preamp out gain by plus or minus 6 and 12 db. Having now used them it seems adjustable gain should be an industry standard. IMHO of course