I have used the Van den Hul IC’s for about 10 years and I am very happy with them.
I have used the The Bay C5 HYBRID, D-102 mk III, First Ultimate mk I and the mk II. My favourite is clearly the First Ultimate
mk I. As you probably know the interesting thing with this cable is that it has pure all carbon conductors and traditional metal RCA’s.
The most prominent impression of the First Ultimate mk I is a very effortless, airy and spatial presentation. The frequency response is seemingly flat as a pool table and one cannot really point out any obvious dips or peaks IMO. The cable is very true to the source; you know: garbage in garbage out, and it will not inject life in inferior sources, nor add more (or less) bass or tame digital artefacts: Everything the source produces will be let through, unaltered. You have been warned.
Good news is that if you have a good source, this cable will do it right. I find this cable a very
natural sounding cable. Really great for voices and acoustic music.
Note that I have been talking about the
mk I of the First Ultimate, which AFAIK is not in production any longer. Van den Hul says there should not be a difference between the two, but they differ slightly in diameter, and IMO, they do not have the same sound either. The new
mk II doesn’t have the tremendous bass extension (nt the same as emphasised) of the original. The midrange is also more "polished" to a shine. The mk I could show some fine grain in the sound, but not disturbingly much so IMO, rather the opposite: It renders nice texture to voices. With the mk II the grain is definitely gone. Voices get very clear and crystal like. I like the mk I better.
To summarize it
Van den Hul First Ultimate mk I
+
Very effortless and airy presentation
Expansive soundstage
Perceivably flat frequency response
-
Not an up-front sound, rather slightly distant
A bit dark sound as opposed to lively and with emphasised mid range.
The
D-102 mk III is similar to the First Ultimate mk I in general, but with one important difference. If the First Ultimate was a bit distant sounding; the D-102 mk III is absolutely more forward. It has the same flat frequency response, terrific bass extension, but the whole presentation is moved more closely towards you. The treble and leading edges of instruments are also much more defined making e.g. “s” sound sharper than with the First Ultimate. I use this cable in my modest home cinema set-up and I like it there very much.
The Bay C5 HYBRID is a bargain and basically IMO the budget version of the First Ultimate mk I sonically, though construction wise it resembles the D-102 mk III more I think. The Bay is a scaled down version of the First Ultimate sonically. It has virtually the same character but sound smallish in comparison with less dynamic head-room. Mind you, it is quite sufficient.
Mainstream HYBRID power cord.
I have not tried other after market power cords, so I don’t have anything to compare with. Anyway, when I connected one to the HeadAmp GS-1, which I use for a RME card in my PC set-up, the sound gained a very obvious better bass extension, more vibrant tones, velvetry reproduction, and better microdynamics
The soundstage expanded with more ambient cue’s and better decay of tones and the overall sound got better definition. Interestingly, I connected another Mainstream to the PC, and behold, the sound improved even more but to a lesser extent. I am honestly still in shock of this latter unexpected effect. I probably have very dirty mains. I bought these cables second hand and I consider them a bargain and really a good and fruitful upgrade.
A word of warning about eBay items: Home terminated carbon fibre cables of van den Hul are said to not necessarily hold the same sonic standard. It might be tricky to terminate carbon fibre correctly. I think they are crimped usually.
On a side note. Goredwings mentioned the Nordost sound. IMHO, at least the Red Dawn mk I does not have the effortless bass extension of the First Ultimate mk I. However, it does have a better instrument positioning. It is also a more forward and much faster cable.
EDIT: The negative points about the First Ultimate mk I listed are of course subjective, nevertheless, representative for what people are thinking about them in general. If you value those traits (like me) they are of course not negeative points.