Given the price of a LD mk9, I personally don't think it would be worth it....
I have a 4-speaker system: two satellites and two subwoofers (that I haven't listened to in ages). Out of the preamp, I have an electronic crossover which in turn feeds two stereo power amps, one for the satellites and one for the subwoofers. The crossover point is 100 Hertz. Unless the crossover point is substantially much higher in your system, dedicating a preamp to handle a frequency range of 20 to 100 Hertz, or so, would be a real waste of money, and further, would make your system unnecessarily complicated with no discernible benefit. I strongly suspect that if you install 6AS7's or 6080's in your IV, the bass would likely be indistinguishable from that of a MK 9.
But anyway, as usual, my opinion is exactly worth the price you paid. :)
Interesting setup. I was thinking of a crossover more in the range of 1,500 Hz, but not an absolute active filtered crossover. Relying more on the passive crossover within my bi-amped speaker.
I agree that modded Mk IVs likely sound identical to the Mk 9 with regards to bass, and probably 'fullness', which I see as coming from lower-mids. I'm going to try some 6SN7s first, but I still may do it.