Hey there,
Yeah, sounds like you need to get the BFD working. The IB is a different animal, and has a slightly different sound to it than other subs (to some degree, at least). I also think that I used to get more "boom" out of my old box subs. To me, the IB just has a presence to it. It's there, and it can be subtle (i.e., not boomy), and it blends nicely with the rest of the sound, be it music or movies. Now, when it needs to, the IB can get scary loud and actually shake the walls (seriously, I have never turned mine "all the way up" because it's just too much, walls and windows are visibly shaking and audibly rattling). Anyway, yeah, I hear what you are saying. Now, I've tuned my IB using REW and a BFD and it integrates well and sounds good...
As to USB soundcards,
here's a thread that offers some ideas. I don't use a USB soundcard, so I don't know which one's good and which one's not. Still, I think any reputable manufacturer's model will work. There have been some warnings that some cards don't support Vista well, so if you're a Vista user, that may be a concern. If you're not a Vista user, I think you should be pretty safe as far as drivers go. Also, Vista's been out long enough that there may be better driver support at this time.
As you noted, you'll need an SPL meter of some sort. The Radio Shack models are popular, and we here at the Shack have the calibration files for all flavors of the RS meter. The Galaxy CM-130 is a nicer meter and I'm sure you can find one on eBay or elsewhere online.
You will need to connect the meter to the sound card. Depending on the type of meter, you may need one connection or another. The output of the RS meter is a normal mono RCA plug. The output of the Galaxy is a 1/8" headphone style jack (I would guess it's mono, but I can't quite recall now). You'll feed that cable (from the SPL meter) to the soundcard. If the soundcard doesn't accept that type of connector natively, you'll need an adapter. They will have them at RS. A common one we use is a L/R RCA (will accept the signal from an RS SPL meter) to stereo 1/8" (goes into the sound card).
From the sound card to the receiver/preamp is probably another 1/8" stereo jack to L/R RCA jacks.
Hey, I should have known better, but there's a thread
here that really describes all this pretty well. Let us know if that doesn't answer connection questions.
Good luck!