They actually play low better than the HD3 in my box, regardless of what the FS is. I don't have any explanation as to why, other than they have a MUCH higher X-max than the HD3 and a slightly heavier cone. It is rather noticable though. The HD3 rolls off quite a bit harder and sooner than the HD3c.
There are a few other things I have noticed. They don't have quite as much punch as the HD3 on kick drums. They aren't really sluggish, and are better than most subs on the market, but they won't quite do what the original HD3 did. Some may actually like that though, as I have heard people say the HD3 is too punchy for their tastes. Personally I love the punch of the HD3, so I kindof miss it. Another thing is that at a low volume on the head unit, it seems like the new sub fades away worse than the HD3. For instance, if I turn down the head unit to talk on the cell phone or something, it seems like the new sub becomes totally inaudible, while the HD3 is still audible. Not that it really matters all that much, just something I noticed.