I have been surprised at how many here have found the f'Deck HPF to be so useful even with amps that are not known to have low freq. issues (i.e. amps that don't reproduce down to the subsonic level). My assumption is that its' use is more for boomy room EQ rather than fixing an amp design issue (like a SWR SM400 & others). This got me thinking as to what is the center frequency for the bass knob on modern amps. Here are a few: Shuttle 9.0 - 80 Hz Streamliner - 55 Hz MB2 500 - 60 Hz 700 / 1001 - 60 Hz and then Fusion 550 - 40 Hz TH500 - 40 Hz If Francis's HPF starts at 35 Hz, isn't 40 Hz a bit low for the center freq. for the bass knob? I would think that this control also has a fairly wide Q meaning that there is also the potential to boost signal below this center point. Could one make the assumption that a HPF would be a required tool for any amp that has a bass knob centered below 50-60 Hz in order to allow the bass to fit in cleanly with a band situation? For that matter, what is the reason that some manufacturers select such a low set point?