First off, this is a commercial unit, and one presumes it worked at some point. Likely Roland did not ship products with non-functioning power lights. My point is that redesigning the circuit is not the approach, we just need to fix it.
Is there anything else wrong? I mean the power light is dark, so does that mean it also won't amplify? Or is the LED the ONLY problem?
LEDs MAKE the 2v (varies with color of LED) by their nature, we don't light them by putting two voltages on the ends that are 2v different. HAven't you got one of the diodes in your drawing backwards? Zeners do not zene in forward bias. They break down predictably in reverse bias. As drawn the current freely flows through the zener.
So the bass version we see is different, but do you have similar power supplies? In other words do you have roughly 54v main power rails? And they are made from a roughly 38 - 0 - 38 VAC center tapped power transformer? If so, and you want to change something, why not just steal the LED circuit from the bass version? A resistor, a diode, and an LED.