Replacing the Vibelink Amp is pretty pointless until the root cause of the issue is known.Return the Vibelink and get the 3e Audio A7 with the 10amp power supply. 140 watts to your beastly speakers should be plenty. I think the amp is on sale at Amazon. All in for under $350
This is highly unusual, not impossible but unusual. In most cases streaming services provide a far stronger signal than HDMI ARC. This coul indicate an issue with the Ultra, but if so, it should have been the same with your previous amp.Yes, the HDMI is considerably louder than the Ultra's ethernet input.
Is the loudness acceptable with HDMI-ARC?
By all means, try the optical connection (you'll not be losing any features).
