2.8V sounds like the typical forward drop of an LED, they are current driven devices so the voltage will be dependent on the characteristics of the specific LED. Your LED strips will have groups of several LEDs in series with a current limiting resistor for each group. It looks to me like there are drivers integrated into the LED board, can you find a datasheet for those chips? The DMX decoder likely puts out logic level PWM signals that you could use to control any LED driver with a PWM input, or just use some mosfets to control LEDs powered by whatever voltage you want. This shouldn't be very difficult but is likely to require a bit of reverse engineering.