TOPIC:

Using a 2x4HD now.  Thinking of upgrading to Flex.  I can use my Fire TV remote to control the 2x4HD.  I know the Flex remote does not learn like the 2x4HD does.  Can anyone confirm that the Flex can be controlled (volume up/down/mute) with a Fire TV remote? 

Just to clarify:

You want firestick to program flex volume up / down frequencies so you can effect the flex unit , .. not input volume from firestick, but as if it were another device added (like you can program it to control TV volume or other firestick / alexa devices).

I see no reason why not if you could for flex frequency / model.   What device type did you select within firestick settings for 2x4HD? 

I want to use the Fire TV remote to change the volume on the Flex.

The 2x4HD is a different proposition because it can learn.  My understanding is that the Flex remote can't learn.  For the 2x4HD, I just taught it to respond to my (Vizio) TV remote.  Then set up the Fire TV to simply control the Vizio TV.  This way, the Fire TV doesn't need to even know that there is anything other than the TV.  The Fire TV is sending the IR codes for the Vizio and the 2x4HD is receiving those codes and responding to them (and the audio settings on the TV are set to not use the onboard speakers).

The Fire TV has a list of devices it already knows how to control.  miniDSP is not among them.  It does have some learning ability for unknown devices (if you say it is a "soundbar" of an unknown manufacturer) but that doesn't always result in success.