CharlieC, I tried the B3 straight into one side of the XLS 1500. I had to turn the volume on the amp all the way up (just running on one channel.) Global volume was all the way up on the B3. Could not really find something I liked, so connected the Ampeg SVP PRO preamp again. Only could spend 1/2 hour as I had to work, but I will try again.
With a typical 8-ohm speaker load, the Crown XLS 1500 will only output 300 watts at best with just 1 channel. I am not surprised that turning everything up in that case did not impress. I would highly suggest that the amp be set for bridge-mono operation along with use of the required, specially-wired Speakon cable to properly audition the B3.
IMHO, the Ampeg models in the B3 are stellar and rival the tones I get with my SVT-7 Pro and its tube preamp.
In a recent post, I provided the settings I use for the SVT sim and I hope you are able to achieve the same results as I do.
Please keep in mind that 3 specific "Level" parameters will need proper setting in order the use the B3 with the Crown XLS and be sure that no other effects or nasty patch artifacts are in-effect while assessing the amplifier sims.
Good luck!