So, a bit of an update. Got home from work today after a 3 week out of town stretch, dug out the Truetone CS6, current doubler off the 2 500mA outputs, bam, powers up every time without issue. Best of all, the horrible headphone buzz is gone.
I also received the 25000mAH battery today, so thought I’d try that as well. Perfect! Powers up and no headphone buzz. Played for about an hour and it used up 5%, so I’d imagine you’d get roughly 20 or so hours from one charge, with just the player.
Anyway, cheers and hope this helps others.