All i got to say is LC18. The screen looks sick. Think about that option since you are investing quite a bit of money. I know you have the $$ if you considered 2 passive rads costing $150-$180.
Please, do not hack a beautiful $200+ plus case or leave the top proturding out. Either way I would
You better start off by buying the case and put everything in there to get an idea how everything fits and what you plan to do. When I built my HTPC, placed the mobo, cd drive, psu, hard drives and filled up the pci slots, the case got filled up quickly. Connect all the sata cables, psu connectors, power connector, and Artic silencer and it gets worse. That was was max 4HD's at the time, not 8.
For 8 HDs, external rad is the only way. But not in the subwoofer. Lots of air is going in and out of subwoofer and if you cut a hole in the enclosure it can ruin the subwoofer intended internal design. Sound quality will drop off. Speaker enclosure is fine but outside ambient air. Speakers get hot and the enclosure air temps will rise.
I think 6 HD's is more realistic and spend the money from the 2 HD's and buy bigger drives. WCing 4-5 drives in the middle column and leave a single drive for OS, programs, under the 5" 1/2 baydrive behind the usb and firewire panel. Remove the right side cage and that leaves you enough space for the pump and tubing and a rad. A 120mm rad on the right hand side will fit easily, maybe a 120.2 even.......
Heatercore does fit but the spacing is very tight because it's so thick. Add a shroud and 120mm fan and that gets packed tighter than a DP sandwich.