I would never say buy it as I haven't heard it. Also why so few of this build? Is it the KGST is better? Too many if's and don't knows.
I explained why it's not that popular. KG/Birgir @ Mjolnir-Audio who makes the amps doesn't have unlimited time. He sells what he has in stock...I've talked to him about it when I purchased the amplifier (KGDT). The KGST was in active production and the KGDT was drawn up by Kevin Gilmore and Birgir and then released on Head-Case where they are very active, not here. You will find more info about things there..anyways, at the time he was moving and had as stuff was sold, he would remove it off the site. If you see my post a few back, I quote some stuff where Birgir explains the KGDT -- it is the sister amplifier of the KGST. It is very similar, but has the option of tube rolling. All of Mjolnir-Audio's stuff is excellent, no doubt, and the constant current source design allows your headphones to actually be able to utilize up the 600Vpp / ~380Vrms swings that the KGDT provides. Before I even bought the amplifier it was advertised as having very good control over the 007 MK1.
However, the slight compromise is that there is less current than the sister amplifier (KGST) which was already known, and thus the KGDT is warmer. Again, here's a direct quote:
The amp was designed around the ECC99/12BH7A and the 6N6pi/6CG7 but others will work assuming they have similar specs and the same pinout. The circuit board has a simple toggle switch to select single 6.3V heater or a center tapped 12.6V setup. Now some slight compromises had to be made to accommodate these tubes so the rail voltages are a bit lower than on the KGST and there is a bit lower current running through the output stage. This is done so that the tubes can’t be run way too hard and thus not last long.
The sound of this amp is very similar to the KGST and so is the performance. Great control of even tricky phones like the SR-007 Mk1 plus the ability to alter the tone of the amp with different tubes. 117V or 230V are no problem as the amp has a switch inside like all my new amps.
The amplifier, as all of his work, is very well designed...and it's very similar to the KGST. However, from an economics standpoint and limited time in a day/opportunity costs, the demand for the Carbon (which is $4600 vs. $2200 - what I paid for the KGDT - and the KGST is $3000 for reference) as well as the highest-end amplifiers they make is very high as you can see it if you go on Head-Case or read the company site, as you see with many flagships that are excellent....and if you have the money and setup for it, it's an amazing amplifier no doubt.
But I am
certain he is focusing his attention on building and stocking the Carbon as well as improving designs for the future with Kevin as they often do for projects. You can see that just looking at their activities on the forums of Head-Case..not here.
Anyways, tl;dr -- in my opinion the amplifier lives up to what it was advertised, and has the quality of his other products - build/chassis, component quality, craftsmanship, proper design (constant current source here, AC heaters) and things like easy switchable mains power for 115/230V and the passive XLR outputs to extend or connect the audio chain easily -- all of these things are found in their other amplifiers and that is certainly not lacking with the KGDT.