I still feel lost and somehow I think you didn't address fully address my concern, so I will try to rephrase relative to your reply.
Let's say I do want to give HA-1 exclusive control over the volume, HOW do I do that?
HA-1's driver only allows for 2 settings to be changed, under the Buffer Settings tab:
1. USB Streaming Mode (set to Safe)
2. ASIO Buffer Size (set to AUTO)
No problem - let's see if we can get you sorted out!
I struck out the streaming mode and buffer size - it has nothing to do with volume control.
Let's start figuring out which drivers your music player is using. There are two types of drivers - WASAP and ASIO.
The ASIO drivers support "exclusive control". ...so that's the one you want your media player to use.
Double check that your media player is using this device.
In this picture you can see that the Oppo ASIO driver is being used. (Ignore the red circle highlighting DSD - this is an old image in my photobucket illustrating a different issue)