Welcome!  (I'm relatively new here, too, but love the product and this online community.)

Unfortunately the Microphone setting is not an option in the "Stage" version of the product (see the chart at the lower right of http://www.pianoteq.com/pianoteq4 to see which options are available w/ which versions).  The ability to tweak that is one of the reasons that I eventually decided to invest in the upgrade to "Standard," and I have been very happy with that decision.

The "Stereo" option in Stage is just a preset representing the stereo results of one particular microphone-placement setup.  But in Standard, you can completely customize the mic setup -- placing up to 5 microphones at various points in the 3-dimensional "space" around the modeled piano.  (See the video tutorial at http://www.pianoteq.com/tutorials?play=tutorial2 for an idea of how it works.)  It's a very powerful customization, which I didn't realize fully until I started playing around with it.  Tweaking the microphone parameters can have a huge effect on the sound.  You can then save those microphone parameters as part of a new piano effects preset (FXP).  These days, I play those customized presets almost exclusively.  (My favorite is the "Dynamic_Jazz_3-mic" D4 grand setting from the FXP Corner, because -- to my ears -- it eliminates the narrow artificiality of mid-range notes that I felt in some of the standard presets.  I am not sure (and don't remember), whether you can play FXPs in Stage that tweak variables that you aren't able to customize.

Hope this helps, and happy playing!
Doug

Thanks
Jason