DDJ-400 speaker sound lags behind headphones

I have the DDJ-400 with Rekordbox and am using a PC with an Intel i3 2.4ghz chip and 8gb RAM running Windows 10.

When I try to use my headphones and speaker at the same time, the speaker lags behind the headphones. I have the speaker connected directly to the DDJ using an RCA to aux cable, and some earbuds plugged directly into the DDJ too. At first I had connected the DDJ to a sound bar, but I understand that could be the issue so I also tried plugging the DDJ into the aux port of a portable bluetooth speaker. I have already tried tinkering with the buffer size, including lowering it all the way.  I've also tried plugging the DDJ into two different USB ports in my computer. I tried two different RCA to aux cables as well- a brand new 15 foot one and another much smaller one. Here are some settings:

Audio: DDJ-400 WASAPI

Sample rate: 44100 Hz

Buffer size: 256 samples (5.8 ms)

Master output: DDJ-400 WASAPI: MASTER

Headphones output: DDJ-400 WASAPI: PHONES

Booth Output: DDJ-400 WASAPI: Phones

Does anyone know what the issue could be?

Many thanks,

Matt