They've changed the wording somewhat since the last time I read this part of the manual, but you can see that it clearly states that the rear input is noisier.
OK, this part I missed.
Yup - I tried the front input, and it's much quieter this morning versus previous days - AND the weird frizzy build up/fade out didn't occur at all. Definitely a strange quirk, but I can live with it if that's by design. I'm just worried about my $2000+ investment.
So, although I'm not all that happy that I'll probably have to use the front input for recording to ensure a cleaner signal, I stand corrected! Thanks sambrox!