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You're not going to cancel the resonance (the "drone") by inserting more silencers. The straight-through glasspack-style silencers don't do anything for low frequency tones, and the resonance is caused by the low-frequency cylinder firing rate (CFR). I calculated it out to be about 110Hz.
The stock muffler uses helmholtz resonators with absorptive material plus long path length to attenuate the CFR. Any straight-through exhaust is going to resonate between 2-3k on the Boxster, period.
I do believe there are ways which you can eliminate the resonance using quarter-wave resonators and additional tailpipe length, plus reactive mufflers, but I don't have regular access to a welder nor do I have the desire to throw a bunch of money at the problem. Maybe someday.
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