Tom, thanks for the response. The power handling of both of my 1x12 cabs is rated RMS. So Bergantino 1x12, 350 RMS, Aguilar 1x12, 300 RMS, both 8 ohms. As to the 3rd cab, I'm not sure at this point, however it would almost certainly be an 8 ohm load with similar power rating.
Just so I'm clear, you said:
"When you parallel two 8ohm loads, it becomes a 4 ohm load, which this amp CAN handle in bridge mode. However, adding a third speaker in parallel will drop the overall impedance to 2.67ohms which no amp can handle in bridge mode."
Does this mean that if I'm only running the 2 cabs and I run the XLS1500 in stereo, one cab on each side, then the Crown will see that as one 4 ohm load, not two 8 ohm loads? I was thinking that in stereo each side would be independent, thinking that if I did in fact add a cab then I could run the pair of 12 cabs on one side (both 8 ohms, daisy chained to produce a 4 ohm load) and then perhaps a 15" cab - 8 ohm - on the other channel. Are you saying that this would not work?
Looking at the specs on the website I got the impression that the XLS could go as low as 2 ohms per channel. I must be missing a key aspect of what your trying to tell me (sorry, am a bit slow sometimes)