Dont have a pre-amp...I have a DAC that has adjustable gain on its outputs feeding an "Unified" amp that has 2 inputs, an input selector and a volume control. That is why i was going to put the high pass on the speaker outputs.
I take it “unified” means the same as “integrated” that is, preamp and power amp combined in one chassis. You can put the filter between the DAC and amp but as you mentioned, the filter won’t work for the other inputs. This type of filter only works before the power amp. Filtering on the speaker side of the power amp is more complicated.
Something I did when I had an integrated amp was add a separate preamp in front of it that had two pairs of preamp outputs. In my case the new preamp was a Jolida 5T. I set the volume on the integrated amp as close to unity gain as I could get (like an HT by-pass input would do) then used the new preamp for switching and volume control of all sources. As an added bonus, the new preamp had remote control.