Well, I seem to be late to the party regarding NAT but I will now try to address this interesting brand a bit more wholistically.
For starters, I am a NAT owner. I have a most unusual NAT but it is built to the same massive standards. I have the Symbiosis SE, which is a SE(T) amp. It is single ended by a hybrid with a SINGLE, high current MOSFET in the output stage. The input and driver are tubes. It is massive and weighs a full 70Kg (150+lbs.) and has a huge power supply and heat sinks. It also runs very hot. Like all good SETs it has no feedback but unlike an all tube SET it does not have output transformers.
Like the all tube NATs, it is tonally rich but not Euphonic. Once fully warmed up and on song it is very transparent, highly dimensional and very balanced from top to bottom. Full bodied does indeed apply but always under good control. What it doesn't have to the same degree of the top SETs I have heard is the rounded 3d imaging...in this sense the presence of the output transistor is felt IMO.
In addition to my own NAT, I have heard at length the following NATs: SE1, SE1 Mk 2, SE2SE (2 different versions), SE3 and the Single integrated. Preamps I have heard are the Utopia, the Plasma and the reference phono. I heard these amps on Apogee Grand speakers (passive and fully active) and Hales Transcendence Eight speakers.
One of the early demos we did was with the Plasma preamp and SE2SE monos against the CAT JL2 signature. Both were excellent but the NAT went to the extra dimension of realism that the CAT could not quite follow despite its other obvious strengths.
So what do I hear from the NAT monoblocks (the integrated is good but perhaps a bit too much on the warm side)? They are transparent and resolving but not at the sacrifice of tonality. They are more neutral than many other SETs i have heard and bass is truly impressive. Image palpability is excellent as is soudnstage organization and (importantly) stability. Images don't swim around...this is a benefit of a large, well-regulated power supply. The sound was similar between monos but I think probably the SE2SE were the best overall sounding IMO. My Symbiosis sounds a lot like this but just with less image palpability.
As for direct comparisons with my NAT Symbiosis and other amps, well my NAT is better than a lot of the competition we have put it up against, including Lamm M 1.1, Octave MRE130 monos and VAC 30/30 Mk3. The one amplifier that soundly beat my NAT is the KR Audio VA350i. It does everything a bit better than my NAT and one thing a lot better. That one thing is the image palpability but bass is also better (surprisingly because it was a strong suit of my NAT) as is low level resolution and as a consequence soundstage depth.
My two friends who had NAT both switched to KR Audio. I used to have a VA 350i as well but sold it for safety when my daughter was born. After hearing one again in direct comparison with my NAT made me want one again...it is really the best sounding amp I have ever owned or heard for sane money.
Let's put it in perspective a bit though. My NAT trounced the Einstein "The absolute Tune" integrated I got immediately after selling my KR (it was a disappointing sounding amp IMO). It trounced the orginal Devialet. It was clearly superior to one of the latest Octave monoblocks and was also better than the Lamm M1.1. I used to own a VAC 30/30, that is actually a very very good amp but being push/pull it could not achieve the overall coherence of the NAT even if it was slightly better in a couple of areas (soundstage depth and image 3d).
This weekend I will hear the large Line Magnetic LM219 ia that is supposed to be pretty impressive...we'll see as I will likely take my NAT to compare with it.
Now that I have a lot of experience with both NAT and KR Audio, I would choose KR Audio. They simply sound more realisitic, higher resolution, better bass (more texture along with the control), nearly perfect tonality, simply the best soundstage and imaging I have heard so far in an amp and from my friend's experience significantly better reliability. So, while NAT is very good, KR from my experience goes about a half level beyond.
It was funny, my friend who brought the KR back over to my place noted that the NAT seems to deliver it all...until you hook up the KR and realize that something was missing. However, when you go back to the NAT it is still very satisfying and you don't really seem to be missing anything...except you just heard that you were missing something.
I have a JJ 322, which by most standards here is a moderately priced amp. However, one shouldn't underestimate it because A) It is made in affordable Slovakia and B) All done in-house (even the transformers). It is very heavy and has huge Double "C" core output transformers (designed and made by JJ). I will be comparing it directly to the NAT over the weekend on my Odeon horn speakers. I have done this before with the NAT a slight favorite but I was running signal passive into the JJ. Now I will perform the demo with two different active preamps: an Audion Silver Night 1.0 and a Pure Sound L10. Preliminary trials with the two preamps on my other friend's Octave monos showed a clear victory for the Pure Sound L10 over the Audion preamp...we shall see how it does now in my system.
NAT is a good deal if you can get them at discounted European prices. The list prices in the US make them look less appealing and I would definitely go with KR if buying in the US. KR is more expensive than NAT here in Europe (real prices not list) so if the budget is somewhat of a concern then NAT gets closer to reference sound than most for a sane price.