FWIW, that heavily reworked M&K V-90 sub was a project that our resident guru
@TLS Guy had a huge hand in helping me with. The front baffle had to be opened by about another 3/4" in diameter to accommodate the SB Acoustics SB34-SWPL-76-4 12" driver, and the rudimentary 90w plate amp was replaced by a Dayton Audio SPA250DSP plate amp. That meant making a custom adapter plate.
The end result is that I have complete control of crossover point, slope, curves, and gain to get it most perfectly integrated and still having the detail quality in that bottom octave of bass that is on a similar level to the Reference 1 speakers.
It handily beat the SVS SB2000 I had, and the KEF Kube 10b's that I still have. You,
@Alex2507 , heard what kind of bass the Reference 1's put out with the subwoofer UNPLUGGED...so you know how important to me it was to have the sub supplement without being intrusive.