Marchand Passive Filter

That's a question for Phil Marchand. My general understanding is that's -3 but I'm not sure. I don't know what sub you're getting but I've found the software in my SVS sub critical to blending. They have an adjustable slope on the LPF that most subs don't 

@mashif 

When you give Marchand the cutoff frequency is that what is considered the -3db point, or the starting point of the high pass?

I just installed one with an 80hz cutoff. It does that job very well but also resulted in an overall improvement of my mains. I don't know if it's the difference in port output or the Impedance change. The filter has a 5000 ohm input Impedance. 100hz seems high but might work for you.