AW: No, the Z-N328 is fine. The reason for the lack of audio output on the subwoofer channel with Spotify as audio source is a limitation of the Spotify SDK. This SDK targets smartphones, tablets and home audio devices and therefore does not have a dedicated subwoofer data channel. This means that the Spotify app on the device does not deliver any audio to the subwoofer line-out, which will then not provide any signal at the outputs.
If you want to drive an active subwoofer or an amp powering passive subwoofer(s), you can use Y-adaptors as workaround, splitting the signal from the front or rear RCA line-outs. If you do so, do not set a high-pass filter for the channel pair that splits the signal to the sub amp or active sub. Another option is to use the high-level signal from the speaker outputs to drive the active subwoofer or sub amp.
The restriction applies only to the subwoofer RCA line-outs but not to the internal amplifiers of the Z-N328, as the internal amps amplify and reproduce the full frequency range.