I'm also trying to make the best of 'decent budget speakers' with my system, and given your use of a WiiM Ultra, I'm trying to understand your aversion to adding a subwoofer. What's up with that?
I'm using two sets of the [2025 updated] Sony SS-CS5 (M2) speakers for mains, and recently had my Fosi SW10 subwoofer fail. I spent a few days without the subwoofer trying to PEQ the best sound from the Sonys alone before picking up an SVS SB-1000 Pro subwoofer (Best Buy open-box - I can relate ;-).
Because the Sonys struggle to produce much bass below 60Hz, and I've got an interesting ground-floor-Loft-overlapping-stereo-zones acoustical challenge, I've found that adding the SVS subwoofer at a 100Hz crossover with the help of the WiiM Ultra has allowed the Sonys to sound their best by only dealing with the upper bass on up.
The WiiM Ultra's subwoofer config/support,what I bought it for, seems to work well. I like using RoomFit targeting a Flat curve with Variable smoothing... At a 100Hz crossover for the SB100Pro and CS5-M2 mains, things seem to measure pretty well:
View attachment 35470View attachment 35468
So I have to admit that to my old ears, the system is now quite good and I'm wondering what main speaker upgrade(s) would be truly meaningful/imapctful to my listening. I feel I've augmented and tweaked the mass-market Sonys to their best performance, and I'm eager to hear what folks with the audiophile bug are experiencing with higher cost and [hopefully] higher-value equipment. Your ELAC DB63s are well regarded in their price range and I've considered grabbing a pair... but I'll be considering higher-end options as well before my next speaker upgrade (more reading to do on audiosciencereview.com, etc). There's an open-box set of KEF Q Concerto Metas at a nearby Best Buy that are calling to me...