Hey hey,
Cable quality can make a difference but only if there's a problem. E.g. I could hear interference hum with the cheap cable I found in a drawer when I first tried it. A £5 shorter cable with better insulation fixed that. Really not something worth spending serious money on! Again - if the sound is good through headphones connected to the Novus, then your input signal isn't a problem, and you can move on to speaker calibration.
The one thing I would say about the input line - every analog input will have a noise floor, so the trick is to run the input line quite hot (i.e. loud), so that the sound you want to hear is maximally different to the background noise. Then you can use the tiny dial next to the input socket to reduce the input gain so that loud -> normal, and noise -> near-silent. It'll need some tweaking: run the line too hot and you'll experience clipping (the signal is louder than can be converted, so the sound crackles) - you want as loud as possible without running into clipping. Since pianos have a high dynamic range, their sweet spot is a bit narrow.
I would recommend biting the bullet and learn the speaker calibration process. I know it's a bit daunting and audio nerds have a persistent inability to say things that humans could understand... so I sympathise. I bounced off this a few times myself before my stubbornness forced me through. And yeah you need to get a calibration microphone. But if I tell you that this will more than double the quality of the sound your Novus speakers will produce, and you put the cost of a mic in context with the cost of a Novus, hopefully it turns into a reasonable gambit . I went with a UMIK calibration microphone but that's honestly a bit overspecc'd for the need, a cheap budget calibration microphone will also serve you well. Try and find one with a published calibration profile, that tends to be a mark of quality.
The corner placement is kind of the factor that forces your hand here: mine is in a corner as well (common for pianos I guess!) and this set up is uniquely difficult for out-of-the-box calibrations for speakers. Standing waves resulting in muddiness and boominess are guaranteed with this kind of placement. But tbh, even if you change the piano's position, the upward facing speakers will always cause some suboptimal acoustics with the wall behind the piano.
Little aside: the way Kawai have mastered the built-in sampled pianos is really clever in this regard: the aggressive dynamic compression + transient shaping de-emphasises sustained sounds, especially in the frequency ranges of domestic standing waves, so it still sounds decent in most placements. That's an uneasy compromise though because it removes a lot of detail from the piano sound. My conspiracy theory is that this is why the NV12 switched to a "soundboard" style transducer: the larger surface area of a swinging plate is less prone to room resonances than a speaker's point source.
Edit/PS: I have no specific pianoteq setting tweaks that help reliably - the problem you experience is audio-chain level not model level. Your best in-app fixes would be to strongly over emphasise the direct sound and pull the resonances and reverb way back; but I think that's a bad trade, as it robs the instrument of much of its soul.
PPS: some tutorials to help you get started with room calibration (disclaimer: REW is a super confusing piece of software and will make you feel dumb. It makes everyone feel dumb)
https://www.youtube.com/watch?v=HYMQ6M-Z5rM
https://www.youtube.com/watch?v=CuEwoeN7ZJk
