I have tried using OEM Seas driver, similar to your Seas MCA15RCY, however the sensitivity is only about 86dB, I have MTM configuration with a dome tweeter in the middle. The sound is not bad quite good better than using Tangband 871 in MTM configuration. I found the Seas drivers sound to be open but the dynamics are fairly limited. I change to a different drivers(cheap), GRresearch mid driver at $25 each and replace the dome tweeter with compression driver x-over at about 2.5Khz. The total effciency is about 95-96dB. The combo is very dynamic, much better than the Seas and dome combo. I forget to add I use like you twin cheap 12" woofer below the MTM. The sound now is very open and dynamic, more tweaking is required I will stick with these drivers.
You want to stick with yr Seas driver I suggest you use the comp. driver(CD) and compatible horn, the combo costs me only about $60, sound is very good, smooth, open sound stage and very dynamic. If you want I can provide details. The lowest recommended x-over is 2.5Khz, you can purchase more exp CD, with lower x-over.
