15x45x9'!!! Even if the actual area is closer to 25' that's still a lot of air to pressurize. The problem I always had with subs is they didn't fill in the lowest octave of the male voice. The ls60 or if your intent is to stay passive the R7 or Ref3 would be the minimum. I've found Kef uniq speakers really respond to power.
I would hope the R7/ls60/ref3 would do a better job filling the mid and upper bass br4 handing off to sub/s.
Regarding the MoFi, or even Tannoy, these speakers do address the bass limitations, but the mids won't sound the same, better, worse, or sideways our OP chose the ls50 specifically for what they do well. I haven't heard the MoFi, but can say with confidence Tannoy concentric with a tweeter in the VC of a 10-12 or 15" woofer sound nothing like a Kef ls50.