The 688 is a wonderful speaker and I would love to own them.
The bass is quite strong with for a bookshelf speaker, although it will never be a floor-stander. That being said, once it breaks in I don't think you will have any worries.
You can break them in with playing regular music, pink noise, or for the fastest way, place the faces very close to each other (say 3/8 gap and reverse the polarity on one speaker, they play them at about 85-90db for 200 hours).
I usually just listen to my speakers and then play pink noise off my ipod when I go out, with in a month I imagine they would be broken in.
P.S. I would love to see some photos of them.