No, there are too many factors involved, as you put it. You don't actually say where these interconnects are destined to be placed in the system, but I guess I can assume it's in between the CDP and the preamp/integrated/receiver/processor/whatever. Regardless, you're going to ultimately be listening to these wires through your amplifying electronics and speakers. Try the headphone comparision you propose, and I think you will quickly come to understand that there is almost nothing in common between the way your 'phones sound plugged into your player, and the way the same disk sounds played through your system.
In fact, judging what is "neutral" is a loaded question from the start. Since your source material, player, electronics, speaker, room, and even your ears (not to mention your headphones) are all *not* neutral, there is no "neutral" baseline from which to judge the interconnects.
Instead, what you need to do is audition the interconnects in exactly the placement and manner in which they will be used, and form your opinions based on what you hear and prefer. Since you say they give a distinctly different presentation, this should not be too hard. Just try to use a variety of the best recorded, most natural source material you have, and audition using both A/B comparisions and extended listening sessions.
Listen for which one sounds most lifelike in your system. Some individual factors you can try to focus on include extension of both bass (weight and power) and treble (airiness and openess), eveness of response (nothing sucked out or sticking out, or top-to-bottom coherence), bass definition and detail, treble smoothness (freedom from imposed textures like grain or glare), image focus and separation, image roundess or 3-dimensionality, soundstage width and depth, articulation of transients and decays, resolution of fine details, dynamic freedom on peaks, ability to transmit very low-level info without veiling, and a general lack of 'electronic' sounding artifacts on well-recorded acoustic instruments and voices.
The most noticable properties will be overall tonal balance trends and extension, but listening for the spatial, transient, and dynamic qualities, as well as textural purity, will tell you a lot about ultimate fidelity to the source, because these factors will let you know which cable is less guilty of smearing, constricting, blunting, contaminating, or otherwise distorting the signal. As I said, the overall perception of "neutrality" will have a lot to do with the balance of your system as a whole made of imperfect constituent parts, so while you want synergy, don't get too hung up on achieving complementary compatability at the expense of other factors that can have as great an impact on ultimate transparency. I would personally rather have an IC that reveals imperfections in my source material and gear than one which glosses them over.
