How I raised gain in my Vandersteen 2W subwoofer
The signal that the subwoofer gets from the main amplifier is line level by virtue of the high input impedance in its plate amplifier. So, i devised a gain
stage for its input. Simple, I used my TVC magnetic passive preamp with 6 db gain option. Installed 50k ohm resistors across the preamp input so the signal is converted to line level before getting to the sub, and wired its output to the sub input. I get 6db more output, not much but readily noticeable with my DeCapos. The issue being that the 2w model's gain goes up to loudspeakers that are 92 db sensitive, which is the actual sensitivity of the DeCapo's, so there is not much variance there. Also the 2w does not have the Q control that could have helped. Furthermore, the DeCapos are monitors and do not really mate well with the sub, giving light to the theory that the 2W subs are not optimal for non full range speakers.
I corroborated this with a pair of full range speakers which really made a big difference with the 2W.
I guess this could be done with any gain stage you might have, it would be a matter of crossing a high value resistor across its input and connecting the output to the 2w input, with a devised terminal for the proper connection.