Adding a Subwoofer to Khorns

You will need something very fast. JL Audio's Fathom 113 should be a great match. I've not heard them paired of with Khorns, but have heard them with Avalon Indras and a Hovland VTL combo and they are killer combo there.

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.

Try a small, but powerful (built in 1500 watt amp) , sub like the Sunfire True Sub Jr. Very small, but very fast and deep bass. Just my two cents...

In the bass region, the room dominates the low frequency presentation. Relatively minor differences between quality subwoofers pale in comparision to the large peaks and dips that the room imposes. So my suggestion is to consider the room to be very much a part of anything you do in the bass region. The approach I prefer is distributed multiple subwoofers, as advocted by Earl Geddes and Todd Welti (leading researchers in the field).

I suggest three or four small subs, with 4th-order low-pass filters that can be set as low as 30 Hz, scattered asymmetrically around the room. Each will inevitably generate a unique peak-and-dip pattern at any location in the room, but the sum of these multiple dissimilar peak-and-dip patterns will be smoother than any one of them would have been. And "smooth bass" is "fast bass" - it's large, usually room-induced peaks in the bass frequency response that make a sub sound "slow".

Duke