Hi Tvad,
I have a friend who owned Silverline Reference 17's, and they were very nice little speakers. In his set-up, I'd estimate they got down to maybe the upper to mid 40's.
I'm skeptical of the 17.5's implied claim of extension to 32 Hz (although to be precise no plus-or-minus-dB tolerance is specified). Assuming an 88 to 90 db efficient woofer in a .65 cubic foot vented enclosure, the response will be down at least 10 dB at 32 Hz without significant boundary reinforcement. While it's true that I don't know the specific parameters of the Dynaudio woofer used, enclosure volume and efficiency and bass extension are interrelated enough that knowing any two will allow a reasonably reliable extrapolation of the third.
You might find it useful to make calculations of your own to check out manufacturer's claims. Speaker designers are pretty much constrained by the same sets of rules, unlike writers of ad copy.
You can download a pretty good little speaker simulation program
here. Try the free demo version - that's probably all you really need. I can help you run a few practice simulations if you'd like.
Best regards,
Duke