Class D amp, rather than digital? D does not stand for ‘digital’ in class D, it’s a pulse width modulated amplification system.

What current draw does your Pandora have? 6W of class D amp output is about 6.6W input power, or just over 1.3 amps at 5v. That leaves about 700mA at 5v for the Pandora from a 2A 5v USB output. If your rechargeable power block has two outputs, it might be best to power the amp from one and the Pandora from the other just in case any current limiting kicks in when you turn it up. I’ve tried reading the Pandora manuals but they don’t state any current draw values, but a digital processor is going to consume a fair bit I’d have thought.