No!! The pre out are connected to the power amps, not the transformer. The 7705 won't make any difference as the 3500's preout voltage is much more than enough for your applications already. The 7705 has the same preamp vol chip so it has the same voltage limit, but it could do better if measured under conditions that won't like happen using real world material such as movies and music and even if that potential issue materializes, it does not apply to your set up in the small room.
Your uneven bass response absolutely cannot be fixed by upgrading to a 7705 or 8805. It is most likely due to the room, you can try using just one sub, stack the second one on top, or continue to explore different locations. At the end of the day, if it sounds good the way it is, leave it alone and enjoy the music.