Try washing your hands better and dry burn your coil prior to wick installation. You might want to use vinyl exam gloves when you wick your atomizer, as rayon wicks very very well and tends to strip oils off of your fingers, which is likely the source of your off flavors.
How long did you you vape on the atomizers you wicked using rayon? In my experience, the off flavor goes away after 2-3ml of juice (at most).
It sounds like the issue you had with your Theorem was you didn't use enough wick. Rayon is very resistant to burning provided there is enough of it to maintain its density after a few firing cycles. If there isn't enough rayon in your coil it will collapse and slowly start to singe away.
Use more than you would think is possible and keep your wick free of contaminants, you will have better results with rayon.
This is really useful info. I had no idea that Rayon stripped the oils from your skin. I'll make sure I wash my hand really well from now on.
I personally have never used anything but Rayon and am perfectly happy with it but it can't hurt to be cautious.