I have quite a few photos of these guys, but I'm also having a lot of difficulty distinguishing between X. crassipes and X. crassicronis, even after having read a lot of the posts. This one from Washington looks a lot like mine, but it's crassicornis:

Not sure how you would tell them apart!

Yes, pictures of the two species can be very difficult to separate. The question is whether the subhumeral band is complete or interupted by an angling yellow-golden patch. Your's is clearly X. crassipes. The two species are similar in appearance, but easily distingished when you have them side by side. Some of the specimens identified as X. crassicornis may be misidentified. Sorry. Characters to look for in X crassicornis are the orange legs and antennae, many (but not all) have the pronotum with some orange to solid orange, the subhumeral dark band is complete, the sides of the elytra are more parallel, and the prominent quadrate region behind the eyes. X. crassipes has the broken subhumeral band, legs and antennae vary from golden-yellow to black, pronotum black, the sides of the elytral are tapering, and the region behind the eyes is not unusually developed.

This is really helpful, particularly the comments on the subhumeral band. Have checked back through my photos - looks as if they're all definitely X. crassipes.