My old speakers, Eltax Monitor 3s, have down firing bass ports, and as I've just upgraded to Wharfedale 9.1, my mum was going to have the Monitor 3s in her front room, but they would have to go up against a wall (like 5cm). So last week before my new speakers turned up, I experimented with putting the Monitor 3s closer to the wall, and the results were quite hideous. I had a thread about it on AV Forums -
http://www.avforums.com/forums/showthread.php?p=3045794#post3045794
As you can see even 2 pairs of very tightly inserted socks didn't bring the bass back down to anywhere near a normal level. 2 socks and the speakers 20 cm from the wall was just about ok, but not any closer.
I don't know a lot about speakers though, it may be that down firing bass ports are worse in this regard than rear ones, and I'm sure foam works much better than socks. Just thought it was worth considering.
What hi-fi point out the same thing if you take a look at the 'compatability' comments in this roundup and look at where the bass ports are -
http://www.monitoraudio.com/pdf/B2WHF206b.pdf