Unlike many posters, I've actually had both cables - at the same time. My Arcam 92 has dual outputs...so, I plugged one run of RCA 1m Quattro Fil into one output, and one run of RCA 1m Acoustic Zen Silver Reference Two into the other output. Both cables were well broken in, and terminated by manufacturer. Tested on a Sonic Frontiers Line 3 driving an EAD PowerMaster 2000 powering Revel Studios. Every thing is very well broken in. A-B'ing was simple, just change the input on the preamp using the remote.
In addition to this, at the same time, I purchased two pairs of each of the cable's XLR 1m versions (for biamping). Of course, it was harder to A-B with these, compared to simply changing inputs using the remote...
So, what did I hear? First, the cables are VERY similiar. They are both easy to like Essentially, the Quattro Fil had far better bass extension and decay in the low-end, less colorolation in the mid-bass, and far, far better decay on the high end. Its true that the Acoustic zen has more coloration - but please please please recognize that it is an EXCELENT wire and this is a minor difference. I definitely would NOT say they were warm, colored, etc...they just weren't as neutral as the Quattro Fil. I tested this using a couple of well-recorded classical CDs, as well as on some modern pop.
I sold all the Acoustic Zen and kept a much more expensive Quattro Fil setup...until I purchased Discovery Cables for my new single-ended tube setup. WOW...you have to try these...forced me to put my Quattro Fils up for sale.
With all this said, I sold my Red Dawn II speakercables and put in Acoustic Zen Hologram II double barrels. A bit difficult to deal with due to the thickness. However, they are outstanding values. I would have loved to put in Nordost SPM or Valhalla, but they were so expensive for bi amping, so I chose the Zen...great bang for the buck, 90% of the performance for much less. Also, after living with the ribbon cables Nordot makes, I will say they are VERY difficult to live with. They are very easy to kink and to expose the wires. Quattro Fil is far easier to deal with...I'll never own another pair of high end ribbon cables that dont have durable shielding.
Of course, I'm also selling Zen these double barrels, too, too, because of my new monoblocks that dont require I use 10' runs...Discovery cables, my new speaker cables, are soooo great...I didn't even consider anything else :)
Good luck and great listening!