I just found that I do have X-Men 2 on DVD here, but it's listed as a DTS 5.1 track. I'll try a few 6.1 tracks tonight, but I don't recall running into any decoding errors such as you describe.

The BDP-83 can't decode DTS-ES internally. It will decode it as DTS 5.1. Of course, with the downmix set to 5.1 it would end up as 5.1 even if it could decode ES. I will toss LOTR-EE in here later and see what I find.

The 983H was bitstreaming audio via the coax to the 950 - very different process. The 950 can decode DTS-ES tracks.