I haven't had the XS since they were first released late last year but from memory, you will definitely get more "slam" and bass extension than HD650. The mids on XS are scooped and it would an adjustment coming from HD650. Not necessarily a bad thing...just different. Hifiman cans tend to have a dip in the 2K region but it was quite noticeable on the XS. Nothing a bit of EQ can't fix. The mids are definitely better on HD650s. Treble is as you described...much more forward and detail retrieval is much better on XS. The one thing I will say is that the soundstage is much wider and open on the egg-shaped Hifiman. That will be something you will definitely notice vs. HD650. At the end of the day, yes, they are different enough to serve as a good compliment to your 650s. I just don't think the XS compared to the HE-500 so I ended up selling them.

I will say this though...I did the Custom Cans mod on the HD650 and it made some pretty significant improvements to the stock sound. So much so that I've sold the XS but still have the HD650 so it says something about how much of a staple the 650s are in any collection.