haunting/dark classical

A few of my favorites.

Elgar's Cello Concerto would be similar in mood to Barber's Adagio and is every bit as good IMHO, well almost. You might also like Barbers Violin Concerto.

I would also vote for Sibelius' Sym #4. For something a little different try Death and Transfiguration by Strauss. It is elevating and beautiful.

Quartet for the end of time is a great suggestion. The Brahms Clarinet Quintet might float your boat as well.

Bartók, Concerto, Fischer Iván, BFZ, Philips or Reiner on Living Stereo,
Liszt Faust, Fischer Iván, BFZ Philips
Shostakovich Symphony no 5, 6, 10, 14,
Mahler symphony no. 2, Fischer Iván, BFZ Channel, Klemperer, EMI,

richard strauss' "four last songs" (esp. w/Jessye Norman! or Elizabeth Schwartzkopf).

Your wanting something "haunting/dark classical" is met completely with the following cd: Gidon Kremer, Hommage a Pizaaolla, Nonesuch 79407-2. For others also reading this thread and after listening to this cd, kindly comment and I believe you will find it as a great discovery, and while some might say it does not fit into the classical genre, I believe you will have a changed mind after listening. When I discovered the cd, I immediately attached the label of haunting to it.