Bass track's pitch seems to vary with my location relative to the speakers

Hello,
I'm creating MIDI/audio music with Sonar, and I have a weird intonation issue with the bass track. The bass pitch I hear during playback seems to go flatter or sharper as I move around relative to the computer's subwoofer speaker. E.g. The bass sounds relatively sharper when I'm sitting in front of the computer screen than when I stand up, and then it seems to go flat by as much as a 1/8 step. I hear similar changes just from walking around the room! It always sounds out of tune at SOME location where I stand or sit relative to the subwoofers, so how do I know if the bass will sound in tune (relative to the other tracks) when other people hear my music on their systems?  Is there a standard way to deal with this?  I assume I'm not the only one to notice this.
My MIDI bass track is Sonar's SI-Bass Guitar (Default). My bass line mainly uses notes A5, D5, and E5 (below middle C4). To my ears, the E5 sounds sharp, so I bounce it to audio and pitch-shift it down a bit. And the E5 sounds ok or flat, so sometimes I pitch-shift it up. But as I described above, it can never sound "perfect", depending on where you stand. And when I listen to the exported WAV on my 2nd computer & speakers, the bass sounds flatter than on my 1st computer & speakers. So what would a professional audio engineer do? Is SI-Bass Guitar known to be correctly in tune, and maybe it just sounds out to me? Or are there known retuning parameters for SI-Bass, to make it sound the best?
 
Thanks,
Gordon

The subjective pitch of bass changes with volume.

Sorry not to get back sooner (very busy with my day job).  I found that if I turn the bass way down on my computer speakers, then what I hear is flatter, and I guess that's the "true" pitch.  When I turn the bass way up, it sounds at least a quarter step sharp.  And with headphones, it's flatter (apparently the "true" pitch).  Also, I had my computer's subwoofer almost against the wall, and when I moved it away (room treatment, as batsbrew said) the tone got flatter, so I guess the bass was vibrating the wall at a slightly sharper pitch.
 
Now I understand better why the bass pitch varies.  But what's weird is that I only notice this happening with my Sonar piece, not with tunes I hear on YouTube.  Maybe it's my choice of Cakewalk's SI-Bass
 Guitar - the default sound, which has so little treble in it.  I'll experiment with other SI-Bass
 Guitar sounds and see if I have less trouble with them.