Audio engine makes a DAC, called the D-1.
It takes a USB connection and converts it to analog with a headphone jack also and a volume control.
That may be the answer to your volume control issue between the A-5's and the sub. Set the speakers the way you like and use the volume knob on the D-1 to raise and lower.
Plus you get pure digital audio, not the 3.5 mm. mini plug output.
If you've already spent $800 on computer speakers, you're not the person to scrimp now.