I don't have a PSR-EW300 -- I have a PSR-E433. But what you are describing sounds similar to what I see on my keyboard and what was discussed here quite some time ago. So, I am going to assume that both keyboards are similar in this regard, but of course, someone correct me if it turns out I am wrong or left something out.
When you say that you are using the voice of a grand piano, then I assume you mean that you've selected the main voice of the keyboard to play as grand piano. And yes, with ACMP (auto-accompaniment) mode on, the main voice will not be the voice you hear on the left side, or accompaniment side, of the keyboard. To alter the sound on the left-hand part of the keyboard, you need to use the split voice, which I will describe in more detail in a moment.
Basically, if you switch ACMP mode on (auto-accompaniment), but do not actually start a style playing, then anything you play in the left part of the keyboard (the part where you play the automatic chords) is going to have a strings/organ type sound. It doesn't matter what style you have selected -- I tried over 30 styles on my keyboard -- you get the same sound with each one. You only get the variety of left hand sounds when you actually start the style playing, and that's when the various pianos, guitars, strings, bass, and other sounds are played in rhythm based on the chord (or easy-play chord selection) that you are playing to make it sound you have a whole band backing you up. And on these keyboards, you cannot change what voices are playing with the auto-accompaniment style. They will vary depending on which style you select, but for any given style, you cannot alter the auto-accompaniment voices.
At this point, I will now point out that I had to majorly edit my post, because as I was trying all this on my E433, I just discovered something I never knew before. You can combine the split voice and the auto-accompaniment mode -- which I already knew -- but what I did not know is that, when auto-accompaniment mode is ON and the style is NOT on and running, if you combine this with split mode, then you WILL get the split voice sound on the left hand part of the keyboard, and it WILL follow the auto-accompaniment easy-play chord feature. By that, I mean that, with auto-accompaniment on and combined with split mode, if you select the split voice as, say, a piano, and then only hit a single C note in the accompaniment/split part of the keyboard, then the keyboard will treat that as an easy-play chord and play the whole C-major chord of the notes C, E, and G with the piano sound. You will also get a bass note, of which I currently see no way to turn off. And, in this case, the sound you select with the split voice replaces the organ/strings sound you are currently hearing.
So, try to see if you can combine split voice mode and auto-accompaniment mode with your EW300 and see if this works. If it does, then it should give you even more flexibility when you actually turn on the style, because then, with both split voice and auto-accompaniment engaged, you can play your own chords with your own rhythm to go along with the pre-programmed background of the style. Again, the notes being played with the split-voice will follow the easy-play chord rules (with auto-accompaniment engaged), so this allows you to use the pre-programmed auto-accompaniment and still give the song you're playing some of your own touch and creativity.
Now, if the EW300 does not allow you to combine split voice and auto-accompaniment, then just turn off the auto-accompaniment (ACMP) mode and only use the split mode. Select piano for the split voice, and then you can play whatever you want there with that voice. However, you will not get the easy-play chords -- you'd have to play all chords manually.