Here are some opinions on your latest posts with a few facts to throw out first.
1). I do not own either D's or D2's (as they appear to be called now).
2). I have a desire, but not yet the fortitude to pony up for any Diamond Series.
So, more opinion based on my lower experience, decent room, and equipment.
You have a tough question. I am a huge proponent of the biggest baddest center you can get. OTOH, if you are a movie watching guy, nothing beats a pair (or more) of quality subwoofers. Your Left and Right are, IMHO, two of the best speakers ever made. For stereo music, I could not imagine not letting them run full range (well except for all the discussion about Audyysey needing to be set at small - no one is forcing you tooo though). Also, not much music has content even close to as low as they can play. Movies though have content that really should be played by a dedicated subwoofer.
First answer then, I'd use a crossover frequency of 60Hz. If you use 80Hz, I don't think you will be able to even tell any difference but 60Hz seems a better choice with your large L/R's. I have experimented a lot with this with my very inadequate (whimpy, whimpy, whimpy ) 803S's. I could not tell any difference during many blast-o-tron type movie content sessions.
As per the center. Golden eared pros will tell you it is best to timbre match the fronts as tightly as you can. I do not disagree with this but I would add a few reasons to save a couple bucks and locate the original D center. 1). If you use a good EQ capable Pre or AVR, the EQ process will help get these different tweeters to sound more alike. 2). I too would prefer the newest D2's but is the tweeter $2K or more better? 3). Although I believe the center is critical, I could argue that there is less high frequency content in most movies going to the center. If the movie soundtrack is edited like most, you will recognize that more of the music type content does play to the L/R channel. One example of this is "Marley and Me". Most of the movie is just dialogue and then every once in a while the movie super imposes music, which is all directed to the L/R and also the surrounds. This is not a well made movie as far as audio goes but is quite typical to many movies. So, to me the center should match or come close to matching the mids and treble, especially balanced across different volume levels; more so than an exact timbre match at the highest frequency. (Those who disagree, let the public bashing begin. It's OK as this opinion might be a bitcontroversial.In the case of my room, my center just is not a perfect mid and bass matched speaker. The drivers are different and they are mis matched as to port no port. Now the trebles, to my middle aged ears do match. Just my personall gripe on my center.
Having great subwoofers to me, is a must. Two is probably my highest recommendation 4 even better (Note, I do not have 4 - preparing to experiment with 3 in a 2+1 dual EQ system). I know I have posted that the center is da thing; but for movies two subs properly placed and EQ'd is da bomb.
For $2K, you could approach getting two Rythmik subs (Personally I think they are pretty unattractive but boy do movie guys rave about what they can do). You could get 2/3rd of a JL Fathom. This manufacturer can't stop winning awards. Two Hsu's or SVS's. The list is pretty long.
Your other comment about the port and distance from the wall. I would imagine 1 to 3 freet. 1 being on the short side. Truthfully, I blew it on this as I had never looked at the HTM2D and D2 as to look would get me wanting - I didn't even recognize they were ported; jaelous, I am now. . Wanting begets buying. Buying creates conversations of a somewhat negative bend with my better half. It's a slippery slope.
Only an opinion then, if you will be watching a lot of movie content and want a set up capable of really playing loud and low; I would recommend getting the D center and spending more on subs. This is not one of those definite opinions though as how you like to listen, what you listen to, and how much you really, really want a total front matching speaker set means to you. Nice problem though and good luck.