In order to get rid of the low frequencies, you'll need to have a long enough muffler to capture the long, low frequency wavelengths and cancel them out. That's weight and space....
With this setup, the easiest way to "mask" the lower frequencies would be to unmuffle all the other frequencies to make them less aparent. You could do that by removing the cats (volume increase of frequencies ~750Hz and higher). Otherwise, to quiet everything a bit, you could go to a smaller diameter pipe.
On a stock engine, 1.75" from each of the cats back to a single 2.5" exit would probably be ideal for more of a typical use application from a sound volume and low-RPM torque point of view.
I went with the (2) 2.25" to a single 3" since, on this car, I personally prefer peak power at high RPM. I never "punch it" from a standstill - I would rather have the extra kick when I floor it 5k RPM's and above.
The nice thing about clamps and a cheap setup like this is that it only takes time to change it all around and try new things to suite your personal tastes :smile:
Dave
