I skimmed the Tools4musik test again, and I didn't read anything about output at 40Hz.
The continuous 1kHz figures you cite came from that review.
There's also an FR graph in that review that shows the NU3000DSP to roll off a bit down low. But nothing too serious: it's down maybe 0.3dB at 20Hz. That's why I'm puzzled where you got the 230W/ch/8Ω @ 40Hz figure.