Little tip. You want to tune pixel and phase clocks for the Amiga core to get the best results. Download ECS V2/V3 config tool (it will run without having the hardware) and open 'test/adjust'. Tune pixelclock until you no longer have vertical bars showing in the pattern background and you see a perfect pattern. Then adjust phaseclock until you get the sharpest picture possible. Then you can adjust H/V position until centeted. This pattern can also be seen in the blue whdload startup window when you run games/demos.
This must be done in the default PAL/NTSC :High Res non interlaced mode. Do not auto adjust screen after this. Many other cores uses the same resolution and if you auto adjust you will have to tune pixel/phase manually again.