The reason I tend to lean towards the soul sacrifice option is that it does work well with the existing fluff and is hinted at strongly in it. The whole purpose of the Eldar Spirit or Way stone is to capture their soul to keep it from being "absorbed into the timeless depths of the warp" as, for the Eldar "to enter the Realm of Chaos as a conscious spirit represents the ultimate horror." (quoting the 2nd ed codex here)
In WD 227, part of the fluff introduced is an imperial transcript which asks of the Dark Eldar, "How do they stave off the depredations of the Nemesis of the Eldar, the Great Enemy?"
Obviously, the DE capture slaves for torture, being an example of the depravity of the Eldar before the Fall and how it has evolved since. This is part of why they are "not nice, not nice at all" (3rd ed codex). On pg 42 (again, 3rd ed codex), the fluff talks of "something that fills their race with an utter dread, driving them onto ever more despicable acts of wanton bloodshed and torure; the Great Enemy...there is a desperation about them...as if their very survival depended upon the extremity of the grievious deeds they perform." In the next-to-last fluff from the codex, it talks of "the air filled with escaping souls...the screams of spirits in eternal torment sounded on the edge of hearing." It does speak of the Archon drinking souls, as suggested by Trunkhead, as well: "Spirits unnumbered are distilled in agony and torutre to the peak of exquisite taste to fill the chasm of my soul".
To me, this all adds up to the logical conclusion that DE need souls to deliver to Slaanesh in a vain attempt to preserve their own soul. The more "able" you are, the more souls you can personally absorb, adding to your own strength (i.e. Highlander, again as Trunkhead suggests), but you must still confront the final enemy. Note that in the fluff, the Archon appears to regain a bit of youth as he drinks in the souls, which he does out of "need". Drinking souls=renewing your own=keeping Slaanesh at bay. Torturing your victims in some way, IMHO, invigorates the soul of the tortured, making its soul, upon its release, more effective in enhancing the consumer's vitality.
To summarize my opinion on this, the Dark Eldar are so dark, evil, and depraved out of a desire to create fear/terror/tortures so that the souls they consume effectively strengthen/renew their own souls, delaying the inevitable consumption of their own soul by chaos.