Amber does nothing to the 23pin/15kHz port. It directly outputs Denise' modes - usually 15 kHz.
You can use a TV's SCART input for 15kHz (with an appropriate cable), though some LCD monitors/100 Hz tubes don't like the signal.
When you turn the phase adjust screw on the back, you should see some change in the video. Maybe the variable resistor is stuck at one end? It gives low clicks when the end is reached.
Or maybe something very simple: is the RGB monitor running the same standard as Miggy? PAL output on an NTSC monitor might roll vertically but can usually be adjusted.
How do the lines look like? Vertical, horizontal, stable, flickering, wandering?
