My 9.1's are around 6 meters apart. They sound great but the one think I notice with these that I never did with my Logitech PC speakers is that songs on Spotify, some of them are horrendously crackly, and on Spotify Samsung TV app most songs have degraded quality. Perhaps I need a better amp?
I do keep umming and arring upgrading the amp for extra features like adjustable crossover and better quality.
Six metres apart?! What have you done, put one of them next door? lol
Seriously though, you and each of your speakers should form a triangle where each side length is the same distance. Where your apartment/flat is big enough that the distance from you to each speaker is also 6 metres then fair enough, but they'll never image properly if you've got them at what amounts to being either end of a wide corridor.
Bear in mind too how sound dissipates over distance. This is the inverse square law. You'll be asking the amp to run much harder to compensate. The little Denon claims to output 2x 65W, but that's a bit of a fib. The total (maximum) power consumption from the mains socket is 55W, and since a Watt is a Watt is a Watt, then all any amp can do is convert 240V mains Watts in to low voltage audio Watts without losing too much in the process. Therefore, if the maximum power drawn from the mains is 55W, and let's be generous and say we lose only 20% of that to heat and running the device itself, then we have 44W to split across the two speakers. That gives us 22W per channel max as a very rough calculation.
Relating that back to sound dissipation over distance, if the distance from you to each speaker was 6m, and say you listened at a volume equal to using 20W per channel, then halving the speaker-to-listener distance would reduce the power required by a factor of 4. We could get the same level of volume (sound pressure in dBs) at a 3m listening distance with just 5W/ch from the amp compared to needing 20W/ch at 6m. Drawing 5W/ch from the amp leaves a lot more power in reserve to cope with loud peaks in the music. A better amp would give you more power in reserve
Speaker distance from the back wall and being away from corners is important too. Corners are crap places for most speakers. They're never really happy there. Try to be at least a metre away from the corner for each speaker.
Your 9.1 speakers use the back wall to boost the bass response from the bass-reflex port. The optimum position for them is something like a 20cm gap between the back wall and the rear face of the speaker.
As far as Spotify goes, it uses the lossy compression format OGG Vorbis which does give the audio quality a bit of a kicking. On top of that, Spotify also uses a very low bitrate for the free version of its service. Both those things combined is what
@hornetstinger was referring to. He's right. Crap audio source. Your Denon can play FLAC and also WAV at up to 192kHz/24bit. Try some hi-res audio files and test if you can hear a difference.
Your Logitechs lacked the audio resolution to let you hear what was going on with the sound. They might have been good for PC speakers, but Hi-Fi gear is a whole different ball game.
Forgive me if this comes across as blunt, but at the moment you sound like you've lost your way a little. What I mean is that you're not really sure what it is you want.
On the one hand, you've spent a load of time and a bunch of cash pursuing better audio fidelity. Upgrading from the Logitechs to the Denon/Wharfedale combo, and then going through the process of buying the XXLS400 and finally swapping that out for the daddy of the BK range; all of that is the sort of journey that someone might take who wants to get more from their music.
On the other hand we have your choice of source. If it is mostly Spotify because it's free, then that's your Achilles heel.
Any move to better gear will certainly let the quality of good recordings shine through, but at the same time it will expose the weakness of poorer sources. You can't 'fix' a bad source with better audio gear; only downgrading to worse gear again will mask over the cracks.
Is that really what you want?