In my opinion the answer is - it depends.
A power cord upgrade on DAC can make just as significant improvement as the right power cord would on the amplifier.
If your system is able to resolve the differences a power cord upgrades make I’d say DAC, pre-amp or amp…doesn’t really matter where you start however the subwoofer would be last on the list.
Also, keep in mind that there are power cables that work better on DACs than they do on pre-amps or amps and vice versa.
Just my $0.02.