The Beta heads use C-MOS logic chips to control the channel switching and the LED's.
The most common problem I see on these when one of the LED's does not light up is that one of the C-MOS chips has failed. There are inverter chips, and a boolean logic block chip. The inverter chips are fairly easy to find, the logic chip is harder to find but you can find them on ebay sometimes. A good tech that works on solid state amps and that understands C-MOS logic will be able to fix this easily. Chances are that you are not getting full volume out of the amp as well because the channel switching and led control are in the same chips so you won't get a full on/off signal the the FET switches.
If you in So Cal I can help you, but other than that choose your tech wisely.