S400s are great for smaller rooms and are happy being driven either by an integrated (e.g. SN2) or a good pre-power combo (in my case NAP 250DR).
My room is on the smallish side - 14x16 feet, semi-open and I'm using a 200-watt Plinius Hautonga integrated. For the time the Ovators cabinet size is a bit imposing compared to the relatively diminutive Totem Sttafs, but that's a visual adjustment and can work over time. As far as a sonic fit into my room, the Ovators are appropriate. They do a range of volumes well.
As to their sound signature - my S400s are probably one of the most transparent and fastest speakers I've heard.
No doubt. Love the 400s bass articulation. Tight, fast with great resolution and clarity. The vocals are pretty much spelled-out in front of you, but I'm still not sure I need the closed captioning in my face .
Few observations on runnign in and set up: they need an absolute eternity to come on song - I bought an ex-demo set and it took them nearly a month before they loosened up. Almost no toe-in is required
I've got a used pair - seems a given with Ovators at this time. I spent the first two hours toying with placement and toe-in and feel like I'm in a good place. My wife's completely unsolicited comment of "wow, the sound is coming from the middle like the speakers aren't even there" seems to confirm the speakers are in a good place in my room. FWIW, they're toed-in.
As far as an ex-dem set loosening up? I suspect that's more to do with auditory adjustment of the listener than the speakers.