I researched the UPS you are using.
The output waveform is displayed as "Stepped approximation to a sinewave" in the specifications table of this UPS. This UPS is a simple UPS that outputs a stepped pseudo sine wave. The output has many harmonic noise components. I find this UPS a good choice for use in digital devices such as computer equipment, but not very suitable for use in audio equipment applications. Maybe if you power your audio equipment from a wall outlet, the noise problem will go away.
I want to check the symptoms.
Does the "A buzzing sound or higher noise floor" you say come out only from the DA-8s speaker output?
Does the noise come out even when the DA-8s input cable (RCA & XLR) is disconnected?
About the phenomenon that the sound goes into monaural mode Please try how it changes when you make the next connection.
(I'm wondering about the DA-8s' XLR and RCA input circuits.)
When A90 and DA-8s are connected only with RCA cable.
SU-9 ------- XLR or RCA -----> A90 ------ RCA -----> DA-8s
When A90 and DA-8s are connected only with XLR cable.
SU-9 ------- XLR or RCA -----> A90 ------ XLR -----> DA-8s
When A90 and DA-8s are connected with both XLR and RCA cables.
SU-9 ------- XLR or RCA -----> A90 ----- RCA &XLR ----> DA-8s
As I mentioned above, I'll try taking the UPS out of the equation as well as try utilizing an isolation transformer to see if that helps.
Does the "A buzzing sound or higher noise floor" you say come out only from the DA-8s speaker output?
Yes, the loud buzzing and high noise floor only comes out from the DA-8S speaker output.
Does the noise come out even when the DA-8s input cable (RCA & XLR) is disconnected?
No, the loud buzzing only occurs when I connect the right RCA connector to the A90 when the A90 is either off or in HPA mode. The loud buzzing does not occur with the XLR connection is involved.
About the phenomenon that the sound goes into monaural mode Please try how it changes when you make the next connection.
(I'm wondering about the DA-8s' XLR and RCA input circuits.)
When A90 and DA-8s are connected only with RCA cable.
SU-9 ------- XLR or RCA -----> A90 ------ RCA -----> DA-8s
Only get the actual audio from the right speaker, though there is audible noise like a high noise floor from both speakers
When A90 and DA-8s are connected only with XLR cable.
SU-9 ------- XLR or RCA -----> A90 ------ XLR -----> DA-8s
I get the actual audio from both speakers, but its the same audio coming out of both and I suspect it is the right audio channel playing on both speakers.
When A90 and DA-8s are connected with both XLR and RCA cables.
SU-9 ------- XLR or RCA -----> A90 ----- RCA &XLR ----> DA-8s
RCA and XLR connections behave the same as described above.
Additional scenarios
When the A90 is removed entirely
SU-9-------> RCA-----> DA-8S
No issues at all, audio sounds normal
SU-9-------> XLR-----> DA-8S
No issues at all, audio sounds normal
When SU-9 and DA-8S are connected with both XLR and RCA cables
SU-9-------> RCA &XLR-----> DA-8S
No issues at all, audio sounds normal utilizing either input on the DA-8S.
Using the Bluetooth input on the DA-8S
BT----->DA-8S
No issues at all, audio sounds normal
My old setup prior to the A90
SU-9-------> RCA-----> DA-8S
SU-9-------> XLR-----> Monolith THX 887 headphone amp (not the DAC/amp combo unit)
No issues at all, audio sounds normal from either the DA-8S and the Monolith THX 887