This isn't a terribly interesting find but I thought I'd share:
The Hucard sound generated by the Turbo Duo and TG-CD are noticeably quieter than the sound of a TurboGrafx with the TurboBooster Plus.  More importantly, the bass frequencies are more pronounced on a TurboBooster Plus while the Duo/CD is a little heavy on the treble.  When I discovered this I thought the capacitors in my Duo were failing.  I then tested a second, barely used Duo and a TG-CD deck and both had the same, quieter volume.  The bass on the CD systems seemed to remain weaker even when I compensated for the volume difference.
Obviously, we all have a volume knob so this doesn't mean much.  For those running a CD system, you may want to give the bass a little boost on your stereo if you haven't already.  I don't have a regular, non-Plus Turbo Booster to compare but I assume it's the same.