Obviously, your plan has a problem.
One way out is to send your output to the PA and ask for a return in your amp on stage.
There are a few solutions to your problem. It all depends of the conditions.

What I could do with is a DI box that has a volume control.
We dont have a sound man - just do it ourselves. I guess youre right though I could just get a feed back from te desk to my amp instead

This is not good right from the start. The outputs are not the same, they are stereo. In nearly every sound. You will get different signals in PA and in monitor. But even of it works, how could you control that fast and effektive?
You can either use two or a stereo di box and split both signals, or you get a small mixer, wich can split your signal, for example by using a monitor out or even a FX out. Naturally you can do this also in mono. The left out is mono while you use it alone. As soon as you plug in right also, it becomes stereo. This is a standard in keyboards since decades.
You might get a keyboard AMP or an active monitor to have control over your private monitoring. Without that you need mixer, or you have no individual control.
In theory you could program a PC3 to output and let you mix all that. But you would have to program everything you use to do that. Not recommended. It's a mess.

Since you don't have a sound man, plug your Kurz in your amp and use its effect send to send the signal to the PA. Be aware that if you change the volume of the amp, that changes the overall balance.
My two cents.

Hi Mark - that's what I do now - but as said the problem is - during a set - the on stage sound increases the longer the set goes on (and a drummer that seems to drum louder at te end than compared to te start opf a gig) - so if I then adjust the amp volume - it affects the PA feed.

What I will do from now on is feed both keyboard outputs to the main PA and then use my own aux channel to come back to my amp - that way I can boost my own on stage sound without affecting the main PA output level.

Either that or get a different drummer.......

Keep your drummer.
He probably thinks that the lead guitarist is in cause.
Check with him!!!
Just kidding...

That is about 99.999% true, lead guitar blames drummer - so turns up - drummer then drums louder to compensate, so we then have an on stage sound which is a mess - and there is a sensible keyboard player trying to get a solution.

Electric drum kit seems a good option as well.....

If you use 2 di-boxes or a mixer to split the signal and feed them seperately into a personal AMP and the PA it does not affect either of the two. What is so difficult about that?

In the end I used a DI box from LE mono output - the DI had a normal out - which I send to the PA and a line out / through which goes to my amp - hence signal is split evenly , and amp volume change does not effect PA signal

Thanks for the advices, very helpful.