Dynaudio c2 or c4 - need some help

This is going to be a very problematic room. 16' and 18' are very close together and 16 is almost the double of 7'8". All the modes are going to be similar and the summation of modes means many very large peaks. Given your choices, definitely C2 wtih sub.

Even using equalizer for C2/C1 to produce an even freq response, there will be prolonged decay time the problematic modes.

If it is not too late, change one dimension, maybe do 7'8", 13', 18'.

That's a very reasonable plan. Two subs would be better than one and I think both F110 or F112 would do great.

Double dry wall green glue recipe is great to sound proof the room but does not always produce the bass quality that you like. It will reflect more base than single layer contruction but tough to know whether this will be to your taste. The doors and windows are also large variables in this game. I recently blocked all windows (removal solid panels) and got new 1.75" sound proof door. Bass reverb was out of control. Some windows partially block worked best for me. Very difficult to trap bass freqeuncies under 80Hz.

Certain amount of bass leakage may not be a bad thing and may even alleviate modal problems. No room is perfect.

A lot of this is unpredictable. If possible, measure and listen to the room before the finishing touches and adjust as needed. Leave some room for traps and diffusors.

A certain benefit with your plan is a very low noise floor, which is always good. My room noise floor was about 50db depending on wind. After new door and window panels, it is into the 40s. There is a tomblike silence which expand the dynamic range of the room without busting your ears. A very quiet room needs more attention to diffusing slap echo to good effect.

Good luck on this long journey. I think you will like the end result.

Not just the room size but the ratio of the dimensions.

Dan, is it going to be HT room too?

Going back to original questions, the C2/C4 vs C1 would really depend on whether you like the stacked design or not.

The extra woofer towards the top is another bass source. When combined with bottom woofer, theoretically yield a more even in room bass boast due to markedly different speaker-boundary interaction. Whether this is going to work for you is difficult to predict. You may try modeling with RPG room optimizer to see which yield a better response within your practical constraints. Interesting to me that Dynaudio going a marked different driver arrangment with new flagship.

More drivers/larger cabinets will usually yield higher sensitivity, higher jump factor and move more air. Most dynaudio are better off with SS. It's not like you can do tubes on the bigger ones. C1 is cheaper with much easier logistics. C2 better deal on used. All need subs especially HT. For music, you may be crossing over to subs at lower freqeuncies with the larger speakers. It may reduce the audibility/conspicuity of the subs.

You may be able to choose listening position to minimize one to two primary modes. My guess is that would be sitting near the center of the room. You will be pretty close to the speakers which some people like. However, nobody like nearfield TV watching : )