It's the big coils that cost a lot of money. But with the price on sub amps as low as they are why bother? It's also another good reason to make the sub box a bandpass design to get some acoustic filtering. Plus, if you go with a separate amp for the array it's no biggy to integrate a high-pass filter into its front end. Then again, if you're driving it in a 5.1 system the receiver handles the crossover chores anyway. I haven't given much thought to the T line yet, but assuming I want to get it to a quarter wave at 100 Hz it would only have to go about 3 feet long, which matches well with using eight four's.