"Ok, here's a super simple build that will massacre anything available for $300 commercially in both output and "musicality"."
massacre? really? there are several subs with that kind of performance/pricepoint. the bic pl200 has been measured by ricci at data-bass to produce spl comparable to the model of your build. it is the ~$300 category, sometimes less depending on when you check prices.
mathu, the absolute best bang-for-the-buck subs are horn loaded subwoofers. the downside is that they tend to be large, but there are some smaller ones. subwoofers are all about tradeoffs. everybody would like a tiny, low cost, high sensitivity, 10hz bone crusher, but they do not exist. most commercial subs are ported and tuned in the 25-30hz region, with internet direct models coming in with lower tunings. $300 is a tough pricepoint to beat what you can buy commercially if you go with a standard design.