Subwoofer - thoughts on Rythmic F12

In a word, Yes. I have Rythmik, SVS and REL subs in my system and I am currently considering upgrading to all Rythmik subs. The Rythmik is a little tighter than the SVS and much tighter than the REL, which is due to the servo in the Rythmik instantly stopping the driver which greatly reduces if not eliminates overhang or blurring of the musics structure. The servo also helps the sub cycle lower with less rolloff.

Beware that the servo puts a lot of stress on the amp and the driver so you will need to make sure that the heat created by the amp has a place to go and that cone stiffness is of primary importance.

The Rythmik may help a little bit with the booming issue you are having, but that is primarily a room issue, best addressed with acoustic treatments, e.q., multiple subs or all of the above.

The mushiness is probably a frequency overlap issue between the sub and your mains and you should be able to correct that by lowering the the lowpass filter setting on your sub or by adding a high pass filter to your mains

In my opinion and experience, you are better off using a line level input. A speaker level signal will have a little more distortion and it will be converted back to a line level signal by the subs speaker level circuitry anyway, but it will still contain the distortion created by your main amps output.

If you noticed more detail in the bass notes after disconnecting your current subs it maybe because you have the low pass filter on the subs set too high, enabling too much frequency overlap between your subs and your F208’s. The 3 db down point on the F208’s is 31 Hz, so you might be able to set the low pass filter slightly above that, but more than likely it will have to be set below that, which unfortunately doesn’t leave much for your subs to do. You may want to consider high passing your mains which would lessen their work load and also reduce unwanted cabinet vibrations created by F208’s low end drivers giving you even cleaner and more detailed sound from your mains.