That's a clamp, consisting of the wooden knob and the flat black piece of wood. Behind the clamp are two flat copper plates, which are the + and - connections. You turn the knob and that tightens the speaker wires against the copper plates.
So you're pretty much limited to clamping bare wires to those plates, or maybe some kinds of very flat spades. I guess the design is in the spirit of keeping everything minimalist.
My speaker wires consist of good Teflon-insulated wires with bare-wire ends. They're inherently warm-sounding, and if they lack detail on the high frequencies, it's not noticeable, because of course the high frequency response of the speakers is rolled off. (It made me realize that a good many speakers actually put out a good amount of high frequency grunge, whether the grunge is from the source or from the speakers themselves.)
