Difficult chamber music

Hi Knowledgeable ones. I have a yearning to get some string quartet and chamber type music into my collection. My tastes tend to run to more of the SideshowBob end of the musical listening scale and I know there must be classical stuff that fits the bill, I just don't know where to begin. I have the Bartok String Quartets and the Fitzwilliam six cd Shostakovich set and some Kronos stuff that I like, but I want some really meaty stuff. Any ideas, I know there must be great music out there, just point me at it, please...

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