biannuzzi - here's some background that might be useful.
The CS3 and its replacement the CS3.5 share the same enclosure volume and woofer, except for addition of an applied damping compound to the 3.5 woofer, which you can get from Rob. In my opinion, the quality of the native bass is superior to any subwoofer, unless you drive it very hard in a big room and need the sub. That's the purpose of the 40Hz bass cutoff. FYI: the unequalized -3dB point is 80Hz, falling at the sealed box 12dB/octave for critically damped, authentic bass response. If you have or ever get a 3.5 equalizer, it has the same CS3 parameters, but better circuit design and implementation. I will have an improved EQ later this year.
Many integrated amps have a 'tape loop' for sending a post preamp signal to be recorded (or equalized) before returning to the poweramp input. The equalizer can be inserted into that tape loop with some small signal degradation.