Has anyone owned both units or spent time with them to be able to tell the difference.
I own the IV Signature. Bass weight and control are impressive. I have not heard a III, but the consensus seems to be that the difference is the bass.
In Sam Tellig's review of the MKIV, he said this:

The MK IV, with its tighter bass, seemed to have a tonal balance different from its predecessors. And it did. Maybe Richard Bews is right : "The amp has more 'richness' now.

At the beginning, though, there seemed to be less of the earlier versions, light and life, which I so loved. I didn't hear the same illuminated-from-within quality that I heard with the MK III.

I didn't hear it for about a week, the change, when it came, was dramatic. The amplifier opened up in all its glory, and I do mean glory. Air, ambience, sweetness, light, extraordinary low-level resolution that has me thinking that so- called 'high-resolution' downloads are probably a waste of money.
hallo, have owned the lfd mk3, and have now the mk4 le signature.
-they are very different, at least it is what i think,
i still do think that the mk3 is the best deal ,less bass weight and depth , but the bass there is is also from good quality.
i also think sam tellig has it wrong there while comparing this two amps. i still think the mk3 is the better amplifier from the bass up, the mk4 does sound more complete indeed , but it also sounds more like other amplifiers in the market(having said that is still problaly better than 90% of them at any price), the mk3 is very,very special in the way it illuminates the music from within like the best single ended, or as a matter of fact the best equipment,(witch is not a lot),and indeed like sam tellig also says....I just think he got it wrong saying the mk4 had the same openess and capacity from iluminating the music as the mk3,.i also had the two of them home, and could compare them ,i really think that , the just named quality it aplies just for the mk3.
witch makes even more special,i acn also tell you that if i get the chance one day to get one second hand , i sell my mk4 signature , and will be more than happy, in fact happier, the rest of the money i willget better loudspeakers.
just for curiosity, i have had at the same time also a musical fidelity kw500, heavily modified ,and i do meam heavily,(via john sampson/ex worker and service engineer from musical fidelity),witch also had no chance against the lfd mk3.
this is only my opinion,and like everything is relative and very personnal, but if it was me, i would save the money and buy a used mk3.(i had the one in black diamond with golden knobs).
francisco from the netherlands
My LFD IV is nearly two days from its arrival to my home. Cannot wait to listen with an open mind and hope it's everything everyone has made it to be. This hobby can be quite demanding and I hope this piece takes me off the merry go round for awhile. Thoughts forthcoming.
Corbacho, did you ever compare LFD III to LFD NCSE?
I've compared the LE3 to the NCSE and personally I prefer the LE3 for its musicality and illuminated sound. The NCSE sounds more Hifi-ish to my ears and fails to connect me emotionally to the music in the way the LE3 does. Of course this is my ears and my opinion. Others might hear differently.
I've owned some "big name gear with big VU's" that didn't give me the thrill of the LFD Mk.III sound. Trust your ears not the dealers ears, and if you buy with your eyes over your ears you are doing yourself a disservice. There are a bounty of integrated amplifiers $5K and under with way more visual pizazz that don't sound as good as the LFD Mk.III. Having owned LFD integrated amplifiers (Mistral, III and NCSE) - I vote LFD's Mk.III as best in sound and value. Always keep in back of your mind that sellers like to sell, distributors value their sellers who sell more the most. I could kick myself for missing buy of recent Mk.III with toggle power switch (original power switch was one functional weakness of Mk.III, functional because I consider a remote controlled integrated as useful but not essential, particularity when some of these "extras" may have detrimental effects upon quality of sound.