How common are rumble filters?

[FuzzBoxVoodoo]

It was very common in car stereo installation to use your impedance chart and adjust the freq to your speakers 2,4 or 8 ohms,how do you know what your dealing with at the output of a given preamp out?

More common than you'd think. Most well designed amps have high-pass filtering around 30Hz or so incorporated into the pre-amp, though few of them make note of it.
As for FMods, they're very imprecise in their corner frequency, as that's dependent on the impedance that they work into, which will vary a lot from amp to amp.

 

[billfitzmaurice]

how do you know what your dealing with at the output of a given preamp out?

Without some sophisticated measurement gear you don't have any way of knowing. It's not the pre-amp output impedance that decides the FMod corner frequency, it's the input impedance of the device that the FMod plugs into.

 

[FuzzBoxVoodoo]

So it is similar in the fact that the load dictates the value,do amp inputs vary that much? I never thought of rating an amp input by impedance.

Without some sophisticated measurement gear you don't have any way of knowing. It's not the pre-amp output impedance that decides the FMod corner frequency, it's the input impedance of the device that the FMod plugs into.

 

[billfitzmaurice]

So it is similar in the fact that the load dictates the value,do amp inputs vary that much? I never thought of rating an amp input by impedance.

Read some power amp spec sheets. They can range from 10kohms to a Megohm. That means potentially a 100:1 corner frequency range.

 

[BBEgo]

So is that to say that these FMOD sub filters may not perform quite as advertised?

 

[bongomania]

So is that to say that these FMOD sub filters may not perform quite as advertised?

"May not" is the key phrase there. Their performance depends on what you connect them to.