Yes, the difference between these is massive to my ears, not subtle at all. SE5 Nebula are already premium quality monitors and to my ears they rival sonically in many areas current flagships, or at least the ones I'd the pleasure of hearing.
For me SE6 Nebula are way ahead and beyond that. They are super technical, super extended in entire sound spectrum, analytical/neutral and can deal with anything thrown at them with such ease it's mind-blowing and with just ridiculous transparency, balance and resolution. Emotions through the roof at the same time.
Drums and synths are sublime, very dynamic and with that resolution and fidelity you can pinpoint and extract exact effects used and/or exact model of instruments musician is using; but so are pianos, cymbals (with that kind of treble extension and resolution!), cellos and guitars and female and male vocals, and the list goes on

. If it's in the recording they also have a very spacious holographic headroom, 3D instrument placement and an abundance of air in between. Microdetails and harmonic afterimage capability of that magnitude- again, never experienced previously, i.e. never realised such sustain/decay was there in my recordings

. They are also fast as hell.
I drive them with iBasso DX260, which is also neutral and transparent, on high gain with hi-res tracks and I've never heard anything else at this TOTL level.