Subwoofers do bring so much to the party if you can integrate them with your system and room well, even though your speakers are almost full range.
Room treatments, are a good suggestion.
Getting your power right is also a good call but this is very dependant on your own situation as to how much benefit you will get.
I would also reccomend you look at isolation for you components. Decoupling is the way to go. Speakers and source components benefit the most. I'm using a mix of isoacoustics, stillpoints and black ravioli in my system. I would reccomend black ravioli if you are starting from scratch for most components but speakers and turntables do like something springy like Townshend or isoacoustics.
Once all that is sorted you will start to get good benefits from well sorted cabling lots of options for DIY here.
Finally don't ignore tweaks such as contact enhancers, fuses rfi/emi control and ground plane reference voltage.
If you want some immediate noticeable improvements get black ravioli big pads for your source and DAC. They will lower the noise floor giving you more details improved soundstage and better bass and tone.