I have the CP88 (and a Montage 6, CP5, and a CP300). When I play out I use the Yamaha DXR 10's for small to medium gigs, and if I'm at an outdoor venue or larger hall I use Electro Voice Live X 15's. I also use the QSC Touchmix 16 for a digital mixer and the keys sound great in stereo through either set of speakers. You may want to use a DXR sub-woofer if you do a lot with a synthesizer but I find the pair of Yamaha DXR 10's are more than enough for most piano/electric piano needs. Check them out on You Tube - lots of good videos on them.

Don't waste your time trying to rig a couple of keyboard amps in stereo. I tried this years ago and in my humble opinion the PA speakers are the way to go. I do have a small Roland KC150 and only use it at home for quiet practice. Hope this helps.

I feel you brother...i have the same issue.

GLORIOUS CP73 sound with a headphone.
GLORIOUS Clavinova sound through when you use the Clavinova through it's own internal speaker

NOT GLORIOUS sound when you use the CP73 through the Clavinova speakers, a Headrush FRFR108 or a Behringer keyboard amp. The high definition, the spreading of the sound, the pristine character, it's all gone.

NOT GLORIOUS sound when you use the CP73 through the Clavinova speakers, a Headrush FRFR108 or a Behringer keyboard amp

I was thinking this is just me. 🙁
I own a CP73 for 2,5 months now and tried two different keyboard amplifiers. First a Behringer, then I spent an additional 240 € to get a Roland KC-400. On the cheapest headphones (literally! a 15€ headphone will do) it sounds as I would want it to. However, connected to the keyboard amp it sounds like a different instrument. I am missing a lot of the premium details (a piano, wurlies, rhodes) and it is definitely not comparable to what you get used to from the headphones. I do not have this issue with my 25 year old synthesizer workstation connected to the same amp, so it can't be the amp, right? Also tested this on my hifi boxes and it did not improve. My Retailers are unable to help during the pandemic lockdown.
Is this a defect or an issue of the stereo ouput?

Any thoughts or suggestions deeply appreciated.

Piano sounds are the hardest to reproduce, and keyboard amps are notoriously bad for piano sounds. (Though I thought the Motion Sound KP-408S was actually pretty good, and much better than their KP-200S that I tried years ago.) You're generally much better off going with a pair of decent quality powered PA speakers, which by design are geared toward extended, flat frequency response. Yamaha has their DXR series. I like the QSC K8.2, personally, or for lower travel weight and/or lower budget, the EV ZXa1 also does a nice job. You'll need two for stereo, and you may need a small mixer between your keybord(s) and these speakers to provide them with sufficient input gain. It doesn't have to be anything fancy, Yamaha makes something as simple as the MG06. Though I might go up to at least the Allen & Heath ZED-6 just to avoid the wall wart.