You do not need a second reducer.
The onboard reducer outputs 300w at 12v (
see ICON video for details), which is ~25 amps. The factory onboard accessories draw ~5 amps. That leaves 15-25amps available. ICON recommends leaving 5amps buffer, so you have 15 amps to safely work with on the factory reducer.
The Ecoxgear soundbars (18" & 26") take 12v @ 10 amps, which falls under the recommended 15 amps and well under the 20 amps.
I also have been running an Ecoxgear soundbar on my own cart for months on the factory reducer with no issues as a real world example.
This post does not consider if you have
other non-factory 12v accessories as you will need to add those amps up, but most likely the sound bar is the highest drawing.