The manufacturer (Wharfedale) would recommend using the DFS instead of the 9.1 for surrounds (ie. use dipoles instead of monopoles).
For that reason alone, I used DFS instead of 8.1. ( I have Diamond 8s)
However, to put your mind at ease, 9.1 would do the job just fine. The fronts and sub and center speakers are far more important than the surrounds.
Out of 600+ DVD origs, I can name with one hand, the number of titles I trully remember and appreciate the surrounds.
Such as: Saving Private Ryan (landing on the beach), Star Wars, LOTR (Gandalf speaks Mordor in Rivendale), THX logo (Robot flying around - Toy Story),
certain DSP modes on my receiver when playing certain music.
Most of the time the surrounds just produce explosions ...so its really hard to appreciate the speaker.
Oh, and for a while, I was using BOSE for my rear surrounds, while having Wharfedales for fronts...and when I finally changed to Wharfedale DFS for surrounds, I could hardly tell the difference
But when I used BOSE as fronts compaired with Wharfedales as fronts, the wharfdales
outperformed those same BOSE speakers at least 2x...day and night difference.
So whether you choose 9.1 or DFS or even BOSE
for your surrounds... you'll be fine
Just make sure your fronts ROCK!!!
And as advised above, try to demo them, see if you can notice any real difference.