I'm not going to go back and look for quotes that we've discussed scores of times over the last 10 years, but I will give you a quick recap on the basics, this once.

Rand himself says in the first few chapters of Book 5, and I quote from memory: "He had thought he was strong before, but Asmodean's teachings were making him stronger."

This is proof that he was not at full strength yet in Book 4.

Secondly, it is stated numerous times in the series that physical exhaustion reduces the amount of Power you can channel. During the confrontation with Asmodean at the end of Book 4, Rand himself states that he hadn't gotten enough sleep in recent days, and that he was exhausted. Even holding on to the Source, was a struggle for him.

Those are two clear pieces of evidence that Rand is significantly below his full potential when he confronts Asmodean in Rhuidean.

Now, regarding the multiplier effect in angreal and sa'angreal:

How does the "additive" effect in sa'angreal explain the minimum strength threshold for their use any better than the multiplier effect does?

Surely, if sa'angreal simply gave you access to a predetermined reservoir of the Power, your unaided strength level would be irrelevant?

I see no problem with the fact that angreal are pure multipliers of One Power strength, as are sa'angreal, with the added requirement that the most powerful sa'angreal can only be accessed by channelers of a certain minimum strength.

The problem with your argument, is that the minimum strength requirement is not in any way a diferentiator between the multiplier and additive models of sa'angreal operation.