Did you also try this with two of the TRS lines connected to ground in the unbalanced connection, like the scope shots indicate?
Because the convenient way to connect unbalanced would be 2X mono RCA to 1/4" TS which will ground the ring connection. So if the ring needs to be NC for best unbalanced operation it would be good to know. (@Grooved initially asked about this in post #4441 above)
I was sure I tried it yesterday, but now that you mentioned it, I went back to try again. TRS connection on PA5 side: T=hot, R=gnd, S=gnd. Again, into 8Ω and 5W output: