Sorry if ive missed anything but, Is the car still with them? If so they may not have finished the job...saying that, youd have expected them to tell you this.
Id go back open minded, maybe secretly record the convo but dont go in guns blazing...at first, see what they have to say, id actually ask for the old speakers, as youd sometimes do when taking your car to have a service and ask for the old plugs etc etc. Have you physically checked to see that the original speakers are still fitted? At the end of the day, if theyre trying to rip you off or getting aggressive in their stance, threaten to go to trading standards and definitely dont pay. But try to give them the benefit of the doubt initially. (Although like you, i be steaming!!)