I'm considering purchasing the Impact M4-15i Baltic Birch 4.0cuft Midbass flat-pack from DIY Sound Group for the 4 Buyout 15" Subwoofer's I purchased. If I purchased my own 3" ports in longer lengths, would this lower the tune for the box? I would like them tuned to around 25 to 30hz. If this would lower the tune, and given the fact that there are 4 ports in each enclosure, how long would I have to make each port for a 25 to 30hz tune? My end result will be an HT-12 sitting atop of a Buyout 15" Subwoofer, to make a 3 way setup. I'll most likely use a pair of Crown XLS 1002 amps to power each speaker set up, one amp(bridged 700 wattsx1 channel) for the Buyout 15" Subwoofer and one amp(bridged 700x1 channel) for the HT-12. I'll use the crossover settings in the amps to blend them together. I'm thinking about 250hz and below for the Buyout 15"Subwoofer and 200hz and above for the HT-12. Thanks for any help.