I was researching servo subs - and not liking the investment required for '< 100hz of bandpass' - nor the 'thousands of voices' yelling 'buy this one, or That one, or this one!" - and had been looking into buying or building a B460 when I came across Heathers' posts about her 4641, and shortly afterwards came across a nearly unused 4565C on CL for less than $400, so the choice was obvious, esp with great pricing at the time on the Crown 1502 XLS amps - total investment less than $700.
The 4645C seller told me I was the only one who was willing to deal with the size, and show up with a truck to haul it...and had been on CL for I think a good year before I came across it....so good deals can be had, with a bit of luck.
(For a very brief time I had one heck of a truck subwoofer!)
Aesthetically, yah, I'd prefer a B460, though I suspect the 45C outperforms it ... and couldn't find one in a price range I could manage at the time.
But, had I been more space constrained, or more wallet free, I sure would have hunted down some servo subs to listen to.
It just seems to me, with such fast electronics today, that controlling the woofer cone's behavior by
measuring it should only reduce any of the typical 'large driver' ill effects...esp if overdriven.
And, Philips did it - analog!- 'back in the day', and I was amazed at how nice the deep bass on those tiny speakers was...and still remember it.
Short references here:
https://en.wikipedia.org/wiki/Motional_Feedback
As well as a few threads about it here on AK.
Whether it would sound better or worse in today's subs, I don't know, but would certainly want to find out myself....the space savings are impressive.
What I
can say from experience is that the 4645C sure seems to do 20hz pretty effectively (I have it crossed at 25hz & 50hz, and may narrow that after more measurements!), and has a 'musicality' (?) and such Power & Slam! to it that is hard to believe....literally house shaking.
Also, since I'm driving the 2216ND woofers with a Bryston amp, I have wondered if a 'cleaner' subwoofer amp would be noticeably better or cleaner sounding - the 2216ND is crazy clean & precise, and also very deep...might experiment with swapping those amps someday....
Some big freight trains climb a shallow grade a few miles from my house, and more than once I've gotten up to see what was making the subwoofer 'rumble' like that - but found it was
off. Also does a good imitation of asphalt/street surface grinder/removers a few blocks away.
Anyway, happy budgeting, wallet watching and tape measuring - I sure don't regret it!!
PS. I've noticed that super deep bass in recordings gets dramatically better and better, starting sometime in the 90s...it's really weird to hear stuff you never knew was there before...and also makes most rips from LPs unlistenable due to the very audible rumble in most of the ones I have.
(I haven't actually played an LP since I got the sub....but is on my 'gotta try it' list.) There is a lot of electronica out there that seems to be
Made for Big Subs!!
. It's stupid overkill for my room, but "there's no kill like overkill" I always say 
