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'.

Thanks for the thoughts. Unfortunately, I am pretty stuck with the room (3 of the 4 walls are existing and not really an ability to move the other much without starting a war with then wife (also, the wall i could move in a bit is the 18' and would only get it closer to 16). Recognizing it is far from ideal, my thoughts were to make the room as solid as I could (double dry wall on walls and ceiling with green glue and acoustical ceiling panels) and then use treatments after learning where the problematic areas are located.

also, recognizing the issues, would it be better to have two small subs (for example, JL fathom 110) versus one larger sub.

Im running C4s in my 19-15 room with 9 foot ceilings..no issues with treatment..IMO, I would buy the C1s before the C2s any day of the week.The C2s are a lame duck,large cabinets with no real bass requiring a sub most of the time and resale on the 2s is crap.

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.