You own very good system and I could think that is well mated to your room with some kind of treatment.
Yes, everything is about MUSIC but but we want to enjoy that MUSIC at its best is posible and for this we need the " rigth " hardware.
All two way passive transducers as your Marten has a " problem " and this problem is that the mid-bass woofer handeld at the same time frequencies as low as 30hz-35hz and frequencies up to 2.5khz and this fact means that ( no matters what. ) that woofer develops high IMD and adds high THD distortions. The woofer excursion with frequencies at 36hz-40hz impedes a " clean "/accurated reproduction of the up frequencies and is from this fact that comes that InterModulationDistortion that collaborates to increment the TotalHarmonicDistortions.
" thought I might need a sub...."
Rigth, you need it the MUSIC is crying for. A sub will liberates the Marten from 80hz and below it and will handled the main frequencies above 80-90hz.
When that bass range is reproduced by a dedicated transducer designed for that specific frequencies and that woofer comes with a dedicated amplifer designed to fulfill that woofer needs the overall MUSIC reproduction will be nearer to the recording.
That piano really will " sing " as better than ever.
Now, the Marten are not the only monitors that could fulfill your MUSIC needs, here 4 options that can work really great for you:
https://www.tidal-audio.com/piano/
https://www.evolutionacoustics.com/loudspeakers/micro-series/microone/
https://www.evolutionacoustics.com/loudspeakers/mini-series/minione/
https://www.kerracoustic.com/k300 ( it does not matters " creamic " does not appears. )
the adequated self powered first rate subwoofers could be for all the alternative including the Marten:
https://www.evolutionacoustics.com/loudspeakers/mini-series/minisub/
For me that could be my first step to improve MUSIC reproduction before to think in new amps or anything else.
Regards and enjoy the MUSIC NOT DISTORTIONS,
R.