What high pass filter?

[75Ric]

I've used both the Broughton and the fDeck. Both are excellent.

 

[GreyBeard]

I have a Broughton HP/LP pedal that does the job.

 

[40Hz]

As others have already said, either a Broughton or FDeck HPF would work equally well.

My understanding, incomplete, is that this pedal is band pass. Using the pedal changes the sound greatly, and can be blended in or out as desired. Am I by any chance already doing a sort of high pass when I dial the effect amount downward?

Not really. A HPF drastically attenuates all frequencies below its corner frequency and passes everything else.

A bandpass filter passes a specific range of frequencies while attenuating frequencies on either side of that range. The bassier sound you're getting is because you're emphasizing a lower frequency range relative to the frequencies outside it. It behaves more like a low/high mids boost rather than a bass cut.

Good article on filter basics can be found here. The curves for the various filters can better show what each does:

 

[dbflats]

Thanks 40Hz. I got to thinking after I posted that that it was not the case.