I would hesitate to alter the original design of the speaker, unless I was convinced that I knew more than the people designing them. The designer typically spends greater effort building, testing, measuring, analyzing the results, altering and retesting to ensure their original target design is met.
By altering the output of the lower frequency energy, you risk altering other aspects of the speaker. By trapping the sound energy, you may get cabniet resonances that blurr the image and soundstage.
Anyway, your receiver should adequately drive either speaker. You are correct that the preamp is in need of upgrading.
P.S> a piece of advice from an engineer, I am not sure what a solder is in the kit, or if any recommendations are made. Use as high a content of Silver solder you can find. Pay particular attention to making proper joints.
Regards,
BBIBH
