I was going to comment on the specs of an 8" powered woofer being able to reach 17hz, even with 2 passive radiators helping out, I was in doubt of the specs. They might reach 17hz, but as JohnK mentioned, be down substantially from the rest of the spectrum so they wouldn't even be felt, but they might be able to measure them that low. I would suggest your friend look at one of the smaller subs out there like the Paradigm DSP3100, it has great power and sound for a relatively small box, combine this sub with M60s/M80s and I believe this will sound better and reach lower than the DT's.
