MrGroovy,
You're on the right track with logging the output (though I'd also log side decode, bit reservoir calc, & scalefactor read). Just make sure you don't overdo it and force yourself to wade through too much data at once (i.e., only do one frame at a time).
From 2.png and 3.png, it looks like everything through stereo decode is fine, but everything after it is questionable...
One other thought: Make sure SubBandSynthesis' outer loop reads out polyphase information correctly (looping ss, read element ss from each subband in order, then inverse polyphase decode into your pcm buffer, repeat until all 18 elements have been decoded in all subbands).
Layer III is a bear to get right, so good luck!