At start bass is set at -6 so tweeter is ok. As I increase bass to +6 it starts playing bass from tweeter. So I guess the sound gets distorted correct?
The same with increasing sound at some point it starts doing it even if bass is 0.
What you are hearing is the higher frequency harmonic components of the bass distorting due to the amplifier clipping.
So there is no way around correct?
If you want to listen at a level greater than that amplifier will provide at a reasonable level of distortion (ie.
before it clips) then you need a more powerful amplifier. No amount of "filtering the lows" from a distorted signal will remove those high frequency components. You're not really hearing the bass per se, you're hearing
harmonics of the clipping caused by the bass, occurring at the beat of the bassline.
My preferred way of doing this kind of thing is typically with at least two amplifiers and an active crossover, which has many benefits, although cost or simplicity are not one of them.

Radio signal is horrible and I need dedicated amp and better setup?
The radio signal isn't your main problem, running out of amplifier power is.

Ideally you want an amplifier that is powerful enough to drive your loudspeaker drivers to levels higher than you would ever want to go so that you never get any clipping distortion. While this may be fine in a large PA system with 3000 watt RMS amplifiers feeding speakers that will burn up on more than 1000 watts continuous, letting you still have peak excursions well above what the speakers can handle continuously, that is not always practical in a car.
That's why I tend to use an active crossover on the line level signal, feeding to two separate amplifiers. That way your mids and highs are still always undistorted as long as you have sufficient amplifier headroom, and the less noticeable bass distortion can run well into clipping, generating 10%, 20%, or even higher levels of distortion before really sounding messy, distorted and smeared.
If you use the same amplifier to drive your bass as your highs, you
will notice the distortion as soon as it begins, as you have discovered.

You can imagine how bad it
looks on an oscilloscope when you can
hear it...

(Although, if you're listening to a sine wave, you can
hear as soon as it touches clipping....)
Welcome to the deep rabbit hole of audio electronics! Happy learning!
