I feel the crossover dial on the amp is misleading at best, going up to 150 Hz. Sure it might actually cut the signal that high, but with the kind of response the subwoofer has, as you point out in the review, it's not really a feasible setting either way.
Altogether a decent product, especially considering the price, but it's pretty clear they were working towards a pricepoint.
Could some of the high distortion for the burst tests be due to the amp beginning to clip? 600W sounds to me like a 10% THD, burst/peak rating. At best