I had the similar situation with Line In.
I have Belkin AirPlay attached to this input.
When Belkin was powered from Ultra USB, after i stopped music the input was still active showing music (dancing blue lines) endlessly. I even reported this to Wiim but all the responses were not helpful.
However, when I decided to power Belkin with regular power adaptor, its behaviour and Ultra’s have changed. There is finally autostop function when source is stopped, but it takes like 2 minutes to activate it. Than standby mode clock starts and device goes into sleep (time depending on sleep/standby timer setting).