Is it just me.? It is at 100Hz. To me it has a boomy feel. I took the parametric mid and set it at 50Hz and boosted a little and went wow that's what my passive P bass needs. I am not sure why they chose the voicing they did. I never use the bass control on that head. On my active Sadowsky I boost it at 40Hz and use the parametric mid to season as needed. End of rant.
The freqs used for the tone controls usually reflect the tastes of the designer. Hey, at least there was a way for you to fix it. A lot of amps you'd have to add an outboard EQ if the controls don't work the way you want them.
I never had trouble with the HD350 controls. Of course for my tastes I use very little EQ and usually find 40 or 50 Hz way to low.
My first move with a parametric is to use it to remove what i feel I have too much of rather than peak any one frequency. If you don't do that you might even discover there is even a little lower frequency in there.
A 100 Hz corner frequency is quite common. Where you run into a problem is if your speaker has a response peak at or near 100 Hz. Since a peak at or near 100 Hz makes a speaker subjectively seem louder, many do. It makes for a fine selling point in a music store, not so fine in rooms that also have a resonant mode at or near, you guessed it, 100Hz.
I'm going direct into headphones with a P bass. The 100Hz to my ears is almost overwelming to all the other frequency components as I get when I boost low mids (which I call 200Hz). Great bottom but loose the clarity. When I boost at 40Hz on my Sadowsky pre or 50Hz with my passive P on my EBS parametric, I get a nice stronger bottom that adds muscle thru the whole range but not at the expense of clarity or addition of boom. I suspect the broadness of the filter roll off is what I hear more than the center frequency. I think I understand the Sadowsky choice at 40Hz after some playing around. Dave
Look at equal loudness curves and the frequency response of a PBass and you'll see why. BTW, the main reason why amps were originally outfitted with bass shelving EQ with a 100 Hz corner frequency in the 50's was that the speakers used then were only flat to 200Hz or so. Boosting at 40 Hz would only have caused the cabs to fart out even worse. Nowadays it's just as common to cut at 100Hz as to boost.