Why would you need an amp upgrade to accommodate a subwoofer?
You can daisy-chain the smsl --> integrated amp --> speaker line out -- > sub -->sub amp speaker line out --> bookshelf speakers.
Let the sub X-over define what frequency to hand off to your bookshelf speakers.
So firstly, not all subs accept speaker level.
Secondly, the analogue crossover involved in the sub, no matter how good, will inevitably be noisier than the digital high and low pass filters in the WiiM Amp.
Thirdly, once set up, you can control pretty much every setting for the sub from the WiiM amp using the app on your phone, which is far easier than crawling on the floor trying to access the settings on the back of your sub, returning to your seat, trying to remember if it sounded better or worse before, repeat multiple times.
Fourthly, the WiiM Amp set up involves the Amp, speaker cables from Amp to the speakers, RCA to the sub.
That’s it.
Running through the sub using the Mini/SMSL as a source involves Mini cable to SMSL, stereo RCAs from SMSL to amp, speaker cables amp to sub, speaker cables sub to speakers. That’s significantly more spaghetti.