Brian and Howard, I've seen the Canson BP profile being used in a P600 when the printer was first issued and it seemed fine. I have used the Canson BP profile in the new Canon ImagePROGRAF Pro-1000 where I've observed and measured it's overall performance to be very good. While one shouldn't overly generalize, it has been my experience that when OEM profiles aren't delivering good results, more often than not the telltale signs are obviously shifted colours along with what appears to be moderate over or under saturation relative to the softproof. These are the reasons why I didn't think in this case a poor profile is the most likely cause. Furthermore, whenever one is told that the prints are coming out too dark, whether it is from one profile or another, it is good protocol to verify that the monitor is not too bright. The effect this error can produce may vary from one kind of soft-proof condition to another, so while I would agree perhaps not necessarily the most likely cause again in this particular case, there is every reason to make sure the monitor brightness is properly calibrated for the specific conditions of the printing environment; in this way it is for sure not a contributory factor to the problem.
Another point that may be useful to the OP: to test whether it is indeed the profile regardless of all I said just above, download from the Ilford website and install Ilford's profile for its Gold Fibre Silk paper used in a Canon P600. I have found that these two papers are so similar, the profiles and the papers are for most intents and purposes interchangeable. If the Ilford profile gives you much better prints than the Canson profile, all else the same, then we would have good reason to believe there is an issue with the Canson profile.