You are using the VTF1 in 2 ports open mode right? If not, that is the mode you should use in that situation. 3700 cu ft is still too big for a VTF1 anyhow, you are probably running that sub pretty hard to get reasonable bass output at your listening position. The LFM-1 EX would definitely be a big step up from a VTF1 though, I made a similar upgrade going from a STF2 to a VTF3. I would be looking at a LFM-1 EX or VTF3 if I were you, or perhaps duals of VTF2 or LFM-1 Plus.