Okay, I'll give it a shot.
This is what I understand the stock setup to be:
And here is the mod showing the series connection:
Assuming identical fans, this puts 12V across each fan. There's a nice review of fans for quiet computing at silentpcreview.com (SPCR)
, which might be a good place to start for choosing the fan.
The computer fan here on my desk has three wires: red, black and yellow. I'm almost certain that black is ground, red is 12V, and yellow is the tachometer connection (so the PC software can display RPM). The latter could be left open if there's no tach facility on the circuit board of the Cerwin-Vega.
If in doubt about wiring, there's a fan forum over at SPCR, and the proprietor of the site, Mike Chin, is an audiophile, so I'm sure any fan question, even if non-computer-related, would be welcome there.
Edit: Oops, I should add that you want a 3-wire (DC) fan, rather than a 4-wire (PWM) one. The Noiseblocker B12-2 looks good for noise, but the question is whether the cooling will be enough.