Sorry I'm late to this thread, but I have a lot to say (offer?)
I have used an F-Mod equivalent in the past. They work very well, so I'm sure you will have a good experience with them.
A high pass filter with a 12dB slope works similar to what was already explained, but let me add my $.02. At the next octave BELOW 300Hz, the output will be reduced by 12dB. It is an exponential drop, therefore at 2 octaves BELOW 300 Hz, there will be 24dB less sound, and so on. (It is BELOW 300Hz because it is a high-pass filter, which allows higher frequencies while CUTTING OUT the lower frequencies, thus the diminishment below 300Hz. From what I read, the OP & dhw367 seemed to understand this) The 300 Hz x-cover point is where the drop off of frequencies starts.
As for using a ground loop isolator dwh367, I just had to speak to that point. If you do indeed have noise in your set-up, then you will want to find out why and get rid of it. We used to have a saying in the +12v industry.... "Noise isolators are putting a band-aid on a broken leg". Meaning that while then can help or actually "eliminate" an offensive noise, it is better to find out the cause of the problem and fix it. If you have a ground loop problem, either make a better ground connection or find a better grounding point with less resistance (compared to a "zero ground" point, like the mounting bolt for the alternator). If you have an offensive power wire, then run your +12v feed away from that wire cluster. Not as easy, but it actually cures the problem vs. masking it.
Hope some of this helps. Feel free to contact me if you have any more questions about the F-Mods.
MM