I've carried-on things like ethernet switches which are similar to typical preamps in shape/size/weight... with no problem. As long as you can show that the item can be switched on, they're generally OK with it. Also, the
prohibited carry-on list does not expressly call out electronics of any type (somewhat surprisingly) - but it also includes the following disclaimer which basically states "we'll take it if we want". So no guarantees.
"The prohibited items list is not intended to be all-inclusive and is updated as necessary. To ensure travelers' security, Transportation Security Officers (TSOs) may determine that an item not on the Prohibited Items List is prohibited."