The K-Horn already has a big 15" horn loaded woofer . You have the power to get it moving. Using low power leads to the a common problem of imbalanced tone, which most don't understand. I haven't heard the VTL 125 but you seem to have enough juice to get that woofer breathing in Triode.
I use a 50 watt SS Mac with the same woofer in Klipsch Lascalas. I choose the Mac for the "synergy" Macs have with the Heritage line Klipsch (the real kilpsch.) Other than that I use tube monoblocks with 4 X 6CA7s per side rated at about 80 wpc.
The only Suggestion I have is that you run your amp in PP or Ultralinear modes. With more power you might hear the bass you want. I found integration of a sub very hard, if, you are looking for a mid to upper bass "bump". A sub appears to be best suited for use only in lower octaves. Some say you feel a subwoofer not hear it. I agree and think you might want to try a different speakers if the added power doesn't do the trick.
