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I thought the 600 was the number of passes, but since it's declining, I must have misunderstood. So, here's a screen capture of it.

Good news or bad news first?

I always prefer bad first, so here's my take: that bugger won't be getting any better. Ever. 

Good news: You've attained the highest sample rate I've ever seen!!!! If I see that correct (bit small for my eyes), it looks like you've carbonfluxed your image, twice over! (Carbonfluxing I've read on Lux's site as attaining 10,000 samples per pixel, something I've only done once! The word sounds cool so I keep using it)

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