Haven't heard too many multipolar speakers because I found that I preferred monopolar, direct-radiating speakers for my surrounds. Of the ones I have heard, though, I thought the M&K tripolar speakers and the Axiom quad-polars were very good at creating a very diffuse soundfield while retaining a certain sense of clarity and detail.

I only heard the top-shelf models of each series (Axiom QS8 and M&K Surround 250 MkII tripole), but I have heard positive things said of the lines in general, so I think you should be happy with any of the line that fit within your budget. Some posters on other boards seem happy with the Paradigm ADP dipolars and the Onix Rocket RSS300 as well, though I haven't listened to these and can't give you my impressions.

Victor