VTF-15H gives you the flexibility of going from ported to (almost) sealed with both ports plugged. From what I read here, since the ports are accessible in the front, you can go from sealed to ported quickly.
Do you have an alternate amp, friend's amp to try and feed it an identical audio source. You want to determine if the problem is in your sub's amp circuits or the feed (i.e. LFE from your AMP) from your Denon. You could do this basic troubleshooting to confirm your suspicion.
I agree, buy it now. I'd try to get a price match to get the 3-year warranty unless you can call Denon or Newegg to confirm that it is a 3-year warranty purchase (unlikely because Newegg is displaying an extended warranty purchase with it, so it looks like gray market right now). If you have an Amex or similar, you'd add another year of warranty to your purchase.
I upgraded my 3806 with the 4311ci (I use them both, 4311ci was purchased for HT room).
While the 4311 has a lot of nice and new features, I still like my 3806 audio quality. In some sense they sound very similar, the 3806 sometimes feels a tad superior. That is strictly my opinion, YMMV.
I got mine 2 weeks ago with a manufacturing date of July and it said Made in Japan. I would be very surprised if a 4311ci says Made in China. That would make me think twice before buying my future Denon.