Congrats on the 683's. They are very nice speakers.
I know the HTM 61 has gotten a bad rap on this forum. I wonder how many of the negative opinions are actually from people who own it, heard it or had direct experience with it. And how much is simply herd mentality. Guess we'll never really know. I will concede though that it is an unusual configuration of drivers for a center channel. But it is the proper speaker that the designers at B&W intended to be matched with the 683's.
So I would really suggest you audition it for yourself with your own source material - especially movies that you know very well with some real soft dialog. Most likely you'll be able to hear a HTM 62 & 685 too for comparison. I know the 685 has been suggested as an alternative but personally I couldn't do it - the aesthetics would really bother me. In the final analysis though, yours is the only opinion that really matters. Get the one that sounds best to you! Good luck.