Hello! Yes, you can continue using your DIRECTV streaming remote to control your TV and the Samsung HW-Q990D Soundbar. When the soundbar is connected via HDMI ARC or eARC, your TV will pass volume commands to the soundbar, allowing your DIRECTV remote to adjust volume and mute just like normal.
Just make sure HDMI-CEC (Anynet+) is enabled on your TV for everything to communicate properly. Please note that while basic controls like volume will work, features such as sound modes or bass adjustments will still require the Samsung remote or the SmartThings app.
~ Samsung Solutions