Adding a Subwoofer to Khorns

Looking for recommendations on adding a sub-woofer(s)to Khorns. Has anyone attempted this and gotten good results? Amps are VTL 125 mono-blocks run in 'Triode' mode for 55 watts. Preamp is a Hovland HP-100.

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".

Dear Stickman: Here you can read some info on how adding two subwoofers in real stereo fashion can help in an audio system:

due that the Khorns crossover in the woofer is a " high " 450 hz point IMHO your speakers ( audio system ) is a very good example to achieve all the benefits that a pair of subs could do for your system quality performance.

You will improve in all the frequency range quality performance, no doubt about.

Adding more subs, like Duke point out, makes things go a little better smoothing the bass response but for someone four subs maybe is high on price and maybe we don't have the place to put all those subs.

Now, about what Jax2 point out on " you need something very fast " we can read here something that help to understand that there is no such slow/fast sub performance device:

Regards and enjoy the music.
Now, about what Jax2 point out on " you need something very fast " we can read here something that help to understand that there is no such slow/fast sub performance device:

Interesting read! Thanks for the link, Raul. I frequently find myself stumbling for the right words, and am thirsty for better understanding in the things I enjoy. The article clarified some details for me, in this case - thanks. My recommendations still stand, of course, as far as standalone subs that might work well with Khorns. I used an ACI Titan with Klipsch LaScalas (which don't go nearly as low dropping steeply at just under 50hz) with some success.

I've also heard Duke's speakers at two shows now and his seamless integration of natural, detailed bass (going way down deep) in both those speaker systems was astoundingly good, and of the most memorable sounds at both shows specifically for that detail, as well as just plain great sounding systems featuring his speakers (RMAF & THE SHOW).

Stickman - I assume you are using the Khorn in corners, or false corners, as they are designed for. If that is not the case, that would be the first step I'd take.