Thanks Sean for your comments. They reflected exactly the points I was making here.
The reason I suggested the tape loop methodology was because of Mrtennis' statement, "use the results to help determine which of the two cables alters the sound more than the other." The key word was "alters". I think of this as an objective, i.e., quantitative test. It determines "coloration" and not which cable is to be preferred over another at any link.
Adding another cable to the chain shows up the design of only that one cable. Replacing a cable in the chain is much more tricky as both cables' designs come into play. Here it is not so easy to know which one is more "neutral". They both may be severely colored but in different ways. But one might just "lock in" with the system because this cable is complimenting non-linearities elsewhere in the system.
Because the tape outputs reside somewhat in the middle of the line stage's internal circuit, after the input selector but before the level adjustment, the added cable here would simulate neither the adding of a cable in series from the source NOR in series with the IC to the power amp. But this would quickly show up any cable non-linearities. I can not think of a better way to sonically determine this.
When we insert something in series, we know right away what has been altered. We have added more than a second cable here (connections and switches) but if we repeat this with multiple cable brands/models, we can ultimately determine the differences between each and ignore other alterations that we observed to be common to all. And they could be very insignificant to the cable differences anyway.
Inserting a cable like the Cardas Golden Cross would immediately show how colored this cable truly is. It would score very poorly in a quantitative test. The added connections and tape montior switch here would be moot vs. the changes brought on by this added cable. But such a cable could easily bring on much greater enjoyment if it replaced another in the system....and thus trigger a highly subjective "qualitative" result. A much more linear cable would bring the detriments of the extra connections and monitor switch more to the forefront.
For final cable selection, rather than try to quantify how each cable directly alters the signal, I would listen to one cable at a time in a given link rather than to use the tape-loop approach. This is because I am evaluating one cable to another rather than one cable to "perfection". Each methodology provides a different set of results, quantitative and qualitative, that are valuable to the final choice.
As my system's resolution has improved, I have found that no matter where an IC or PC is used in the system, that cable's effect on the resultant sound is fairly consistent. Only the magnitude of the change is the significant difference. At this point in time, after hearing Purist Dominus affect the system consistently no matter where it was used, and the same for the Kubala-Sosna Emotion, I would deduce that a cable in the tape loop would likely affect the system in a similar manner if used anywhere else.
John