How well does it work with the A5 speakers? Do they also control the sub's volume, or does their output only function as a pre-out.
In other words, can I leave it on in my living room or do I need to get up and tune it every time i need to change the volume?
The RCA output on the original A5 speakers is fixed. The new A5+ RCA output is variable, so in other words you can control the sub volume with the A5+ remote. If you already have the original A5's and want to connect the sub, you can use the source volume control (computer or whatever) and that will control both.
For the A5+ system, the A5+'s will control the volume of the S8 after you initially set the volume and other controls on the S8 appropriately. Once adjusted and connected with the A5+'s, you will enjoy the ease of control via the monitors and not the sub, although you always have the ability to fine tune the sound of the sub.