In my understanding a HPF will actually improve the power output of the amp/speaker combo because it will filter out the frequencies that are either too low to use or are creating mud freeing up the amp/speaker to focus on the usable stuff you are sending it. An 8" speaker does not serve the same function. It will TRY to reproduce those frequencies even if it can't and be robbed of clarity and power handling.
Yes, thanks fingers, that's what I wanted to say, but you did it a better way.
Maybe the speakers are not overdriven (which might damage them) but the resonant frequency of smaller speakers in general is much higher, probably higher than we want it to be, so there might be a low frequency resonance that disturbs the sound. It is common knowledge that loudspeaker should not be driven below their resonsnt frequncies.
It is also easy to overdrive an amp if you put signals in you don't make use of due to the small speaker. To get a loud sound you will get a higher output amplitude with low frequencies than without them, so the amp may clip the higher signal and distort it if it is close to the maximum output possible. Combo amp should filter out low frequency signals below the speakers resonsnt frequency, but I doubt they do that.
Anyway it makes total sense to cut some low frequencies out if we don't want to use them (might not apply to hardrock, punk, techno etc.). Even the bass body does this in the farfield compared to the electrical signal from the pickup or a nearfiled microphone.
As I said, try your pickup or close mic with a mixer and good closed headphones. If you don't know how to change the sound towards your acoustic double bass sound, first try to cut the bass a bit. If possible vary the frequency of the low cut filter. Then, depending on your pickup type, open the mids a bit if needed. Compare this with a microphoned recording in the farfield (several meters away).
Then please tell us what (all of) you found out.
Don't get upset, I just want to help you and show ways to verify it for yourself. You don't need to if you don't want.