Here's something I've never seen before. This review includes a FR graph for the Bose 901 Series IV, which I used to own a pair of. I've seen test reports, including FR charts, of the Series VI, but not the IV.
http://i.nextmedia.com.au/avhub/australian-hifi_reviews_2012_2012-12_bose_inc_cover.pdf
http://i.nextmedia.com.au/avhub/australian-hifi_reviews_2012_2012-12_bose_inc_cover.pdf