Though combining rock and classical music usually leads to disaster, you might want to try 20th century minimalist composers like Steve Reich, John Adams, and Philip Glass.

Hopefully I'm not overgeneralizing, but these composers abstract many rock-related musical concepts--for example, layering and repeating short musical themes to create hypnotic sonic mantras--in interesting ways.

Though I haven't heard Reich's "Music for 18 Musicians," that's one of his most popular pieces. I like "Four Organs" myself, but it's arranged for four organs and maracas. Don't know much about Adams and Glass, though.

The main problem with minimalist compositions is that they tend to sound like theme songs from TV news shows. You have to spend some time with them before the compositions start talking to you on their own terms.

If you absolutely have to hear classical/indie rock hybrids, you might want to try bands like Rachel's or the Threnody Ensemble. I wouldn't recommend them, though. Those bands use cellos & violins quite a bit, but they often sound like minimalism lite.