We can not add sub woofer function for 2 channel mode because it simply doens't work that way.
If you want the sub signal, then you need to use Dolby IIx and configure with other surround and back channel off.
If you find that the sub signal from AVP-16 is low, then you need to get a sub with +12db input. The sub signal from AVP-16 conforms to Dolby standard. I have a REL Storm III with 0db and +12db inputs and I can get house shaking bass from my REL and AVP-16.
We sold out of all AVP-16. AVP-17 is AVP-16 with new front panel and HDMI switch bundle. Will publish photo and spec around end of August. We have no AVP to ship now

If you want to buy a good AVP before we raise the price on AVP-17, you might be able to find one from our dealer's demo unit.