Bi-amp problem: SS & Tubes need your help please

Hi all,
I really need some input on how best to resolve the situation: Audion Silver Knight Premier 2.0 tube pre-amp, one set of outs connected to Musical Reference RM9 tube amp driving 4 Nestorovic type 4's in a head to head configuration. They have an additional built in passive cross over so they are only seeing everything above 250hz. Soubd exceeds my expectations.
2nd set of outs into a Sony MX1000U SS amp with gain controls driving two Nestorovic type 8 bass sub's. They also have additional passive cross overs so that they are seeing only 250hz and below. Here is the problem: I am getting very little sound out of the type 8's. Even with the gain control's on the amp at max and the volume control at max on the pre there is sound but way lower than it should be. I would think with tube pre and ss amp there would to much gain not to little, but do you think it is a gain issue, input sensativity issue's? To further complicate they issue the type 8's have 2 12 inch speakers in each box which is sealed. 6 months ago, I replaced 1 12 inch speaker in each box though close they were not exactly the same as the other speaker in each box, since it is sealed I can see where over time could the different displacements in air cause the speaker to fail and now its acting like a passive radiator? ANy and all help is appreciated.
Butch