I just don't think you're going to get much below 20hz from those subs. Even Sony only rates it down to 20hz, which you're already proven to yourself 2 of them in your room is already -6db. Anything lower will be down even more... And you'd be using what little power it has trying to make it do something it really can't ... probably at the expense of what it otherwise could do reasonably well, i.e. stuff above 20 hz , like the manufacturer states, and you already had someone measure that they just wont do well below that.
I suggest you get a more capable sub if you want to reproduce those frequencies well. Until then, don't try to wring every ounce of performance out of those Sony subs. Let them do what they're capable of.
It has nothing to do with your amp. It is the limit of what those subs can do.
mods, can we have a 'dislike' button ?