Unlike modern speakers, which are often advertised as being 8 ohms but actually dip down below 4 ohms) the 4410(A) are still true 8 ohms speakers. As such, the power output of the WiiM Amp is limited, indeed. Luckily, their high efficiency partly makes up for that. Higher volume would really need higher voltage output in this case.
At least it's good to know that you can crank up the WiiM Amp to 100% volume if needed. It cannot ever go into clipping, that's the way WiiM designed it. Some people shy away from any volume setting >50 because they fear for lack of headroom. No need to ever worry about that with the Amp, turn it up to the max., it will always remain clean.
A sub is always helpful not just to extend the bass but also to relieve the mains and the amp from bass duties. The output impedance of the original WiiM Amp rises somewhat in the lowest octave.
Some speakers are not perfectly suited but it doesn't look like summit really wants to talk about his setup.