The Bose Compact sounds better in mono. Using two units step all over each other, and the sound is actually not as pristine and "all encompassing" as it could be. At lower levels, I can see a slight benefit to the stereo field, but there's just no need for two at lower levels ... one is perfect for most of my rooms. The room I worked last night has too much bounce and the Bose's efficiency is compromised with a second tower.
I knew this, but I wanted to test the waters again, since I plan to take the PA4X out soon. My current setup with PA900 and one Bose is just about perfect for my entire summer ... but I already knew that. The onboard speakers are responsible for at least 40% of the perceived volume in the room, and the Compact just adds fullness and projection.

I found a pretty cool way to monitor my guitar, ipad and voice through the PA900 speakers too: (follow along, now)

I use an iRig MIX (attached right above my pitch lever with Velcro) This is a 2 channel, stereo mixer with a separate headphone out that can be assigned to either channel. The ins/outs go like this:

My mic - direct to Korg VP - out in stereo thru on board speakers - mono to Bose

iPad - into TC VL-3, guitar (same), wireless mic (same) - all feed from stereo out into one channel of the irig Mix, with the volume OFF (gain and EQ are still active) - only the HP out is used, and that feeds the second audio IN on the Korg. This allows my mic, ipad, guitar, and wireless mic all to come through the on board speakers, mixed with the regular Korg sounds. In many places, this is enough volume. The second input on the iRig has the Korg outs - this channel does NOT send the HP out (feedback!!!)

iRig main outs go to the Bose (or any other system) L/R or summed. Pretty compact and versatile, but rather unorthodox. It's like "Hooked on Phonics" ...
Works for me!

I know you'll miss the onboard speakers cant wait to hear more after you gig with the Pa4x a few times,...that's the test.