You need a high pass filter somewhere in the chain. You could:
A. modify the existing crossovers and use switches to put them back to stock for full range
B. get an equalizer and run your source through it before it goes into the integrated
C. put a high pass filter at the input to the integrated. Same idea as the equalizer but simpler. It is something you could make with a few dollars worth of parts from Radio Shack, and if it did what you wanted you could upgrade to better parts. You could either use a switch to bypass it or remove it when you want to run full range.
I'm sure these would compromise the sound in more ways than just lowering the bass level, but it would work.
A. modify the existing crossovers and use switches to put them back to stock for full range
B. get an equalizer and run your source through it before it goes into the integrated
C. put a high pass filter at the input to the integrated. Same idea as the equalizer but simpler. It is something you could make with a few dollars worth of parts from Radio Shack, and if it did what you wanted you could upgrade to better parts. You could either use a switch to bypass it or remove it when you want to run full range.
I'm sure these would compromise the sound in more ways than just lowering the bass level, but it would work.