Striker Symes was sent on after an hour with Orient trailing to Nick Proschwitz's first-half volley.
Dean Cox eventually sent the tie into extra time when his 87th-minute cross flew straight into the net, but Hull squeezed through when Tom Cairney struck from the edge of the area with three minutes remaining.
Slade said: "The only frustrating thing is, and it's just a minority, but when I'm bringing Symes on they are jeering him before he comes on. What's that all about?
"Don't get me wrong, the majority of fans were unreal but you always get the few - we are trying to win a cup tie and all they want to do is jeer someone coming on.
"They might as well stay at home then."