Do I need a lowpass sub filter?

[JorgenMan]

I'm in the planning stages of a subwoofer (to be implemented when I graduate and get a real job). My current plan is to run a ~70L sealed RSS315-HF on a Nady XA-900 or something similar (SAF keeps me from building anything very big), although this is very preliminary. I definitely like the idea of using a pro amp, though, if for no other reason than to save money.

My question is this: there are many DIY subs here which use only an amp and a BFD. Isn't there some need for a lowpass filter to the sub? From what I can see, the BFD doesn't provide this, and neither does the pro amp. The only other thing I can come up with is that the subwoofer output on most modern receivers might already filter the signal. Am I missing something here?

[ThomasW]

Ported subs should have a subsonic filter. The box functioning as a acoustic filter does the job for sealed subs in small boxes.

You'd need to be careful running the RSS315 with one of the Nadys, they have enough power to blow the cone across the room if someone gets silly.

[JorgenMan]

No, I mean something to filter out high frequencies - or is that what you mean?

As for the Nady blowing the RSS315 apart, I know what you mean. I plan on just running one channel and possibly upgrading in the future.

[ThomasW]

Any subwoofer need to be fed a signal where the highs are filtered out. That's what the sub out on a receiver or pre-pro does

[JorgenMan]

Sweet. That's what I wanted to hear.