I had called Panasonic about this and the only way to increase the levels is to hit the LEVEL button on your remote and raise the level... other than that like in stereo mode you can't do it like that.
As for treble, I don't think you can change it, I never found a way to.
You can. I was fooling around with an XR-25 last Friday and adjusted treble, bass and balance using the same set of buttons. But can't remember right now which buttons did this on the remote.
I have noticed an irritating trend of tone controls not working either in non-stereo modes, or they only work on the front main channels. Dumb. Not every recording was made with tender loving care and high quality equipment, especially many rock and pop titles so for me tone controls are a useful feature to make certain music listenable. For example, I was using Groove Armada's Vertigo CD to check out the XR-25 (but it had only some cheesy $300 5.1 speaker package hooked up so no decent audition possible)--that album has a very forward upper midrange quality and on anything other than a boombox, or dance floor filled with people, it is quite fatiguing to listen to. But using the treble control I could tame it so at least it couldn't drill holes in my ear drums. I do the same thing at home on my own system.
Chris: supposedly it took quite a bit of research to get digital amps to reproduce high frequencies properly--how well does your XR-25 do this?