-10 should be pretty loud, although this depends on the source material, too. I assume you've tried more than one source, though.
It probably isn't the wire, but how long are your runs of wire, and what gauge is the wire?
It probably isn't that you're using only one set of posts either...but are the pairs of posts connected? I didn't find a picture of the posts to see how they're connected on the Monitor 50s, but they should be connected somehow.
Did you run the Audyssey auto-setup?