I understand that the KORG HAVIAN 30 has what i needs including the bass chord.
But I highly doubt this keyboard too,
- Is this keyboard suitable for stage performances/music composing - Is the sound quality good and professional?
Sound quality is subjective. You can hear the keyboard yourself online, to get some idea. But being an arranger, demos would often include the backing tracks which (from what you said elsewhere) I don't believe you intend to use. So in videos, pay particular attention to the sound of what the player himself is actually playing at the time (or demos that show solo playing).
Korg's main page
https://www.korg.com/us/products/digitalpianos/havian_30/ starts with the headline "The complete musical ensemble for your home." So I think that tells you that this is not a board they designed for professional stage use. The lack of things like 1/4" outputs and MIDI jacks tells you the same thing. But that doesn't necessarily mean you can't do a professional calibre performance with it.
UPPER 1,2,3 and only LOWER 1? No lower 2?
So i can only layer 1 sound at the left hand after splitting?
I believe that is correct, allowing for the fact that the automatically generated sounds (i.e. "bass chord" bass ) are separate and don't count as one of the three Upper or one Lower sound.
The Casio PX560 is set up differently... instead of three upper and one lower, it has two Upper sounds and two Lower sounds. In addition, any/all of these four sounds can be "hexlayer" sounds which, themselves, allow you to split or layer up to 6 sounds. (That means there's even a way to have up to 24 different sounds split/layered on the keyboard!)
Are there any apps at PC to combine two different sounds/voices and export it back to the keyboard?
(For an example: Strings and Acoustic Bass)
Again, I don't think "bass chord" takes up a playable part, so as I understand it, even the Havian with its one Lower sound can let you have a left hand part of strings (played manually) and acoustic bass (generated separately and automatically from the lowest note by the "bass chord" feature, if I understand that feature correctly ).
But to the actual question, there are tons of sounds you can get from a computer, including sounds that can be better than what are in the keyboards. You can also get more sounds from an iPhone/iPad which can sometimes be more convenient than using a computer, though computers give you the widest range of highest quality sounds.
However, there is no need to export the sound back to the keyboard, you can go directly from the computer to your playback system. Unless you want to hear the computer's sounds through your keyboard's internal speakers. If you want to do that, the keyboard needs to have an audio input. The Casio PX560 has audio line inputs. The Havian 30 does not have a line input, but it may be able to receive audio from the computer via USB, I don't know.
If you want to mix and match a keyboard's internal sounds with a computer's sounds, ideally your keyboard should have certain "MIDI controller" features these boards may not have, so you may run into some roadblocks. But from what I can tell, the Havian 30 is better equipped to integrate external sounds than the Casio is.
Having said all that, I think your fundamental problem in finding what you want is that professional keyboards for stage use aren't generally going to have any kind of "auto bass" system, because professional stage musicians don't use/need that feature.
...If you're playing live with an ensemble, you're probably playing with a bass player, so you don't need any kind of auto bass.
...if you're playing live in an ensemble where you do indeed need to play bass lines with your left hand, your left hand will generally need to play a bass line and nothing else, because "professional calbre" bass lines have their own motion and also need to be locked into what the drummer is doing. Simple bass automatically generated from the lowest note of a chord wouldn't give you the results you need.
"Auto bass" features are really for two kinds of players: home users and "one man band" performers, and you don't typically see "one man bands" on professional stages. Even in the cases where professional bands employ backing tracks, they will use recordings or sequences, not a bass part generated from the bottom note of the keyboard player's chords.
(And even someone using an arranger for a public one man band performance wouldn't be using
just the bass, they would be using other automatically generated instruments as well--drums and other sounds--meaning they'd be using the full arranger/accompaniment functions, which I think you said you don't need. So you're asking for something very unusual!)
If you give up on doing what professionals don't do (auto bass), you'll have a ton of professional keyboards to choose from. But if you need that feature, you'll probably have to live with a keyboard that is less professional calibre in other ways.
