The matching center for the 640i is the DM600IFS. I used to own one, and it matched well with the 610i and 600i I had at the time. You can find them on eBay alot for between $120 and $150. The data sheet/manual for the 600IFS can be downloaded on Adobe format at: http://www.bwspeakers.com/downloadFile/speakerModel/DONE_-product_info_DM600i_IFS.pdf
