I'm not trying to make this harder, but by "simple" do you just want something that works as a crossover, or are you wanting compression/limiting and parametric filters? If you want the latter, its a lot more complex. Do you have a way to test and set it up for maximum performance?
Its possible Community can recommend a processor and/or assist you with suggested settings, but that is a legacy product. With some of the newer passive systems manufacturers (QSC and JBL come to mind) they have processors or processing amps that are pre programed to match the loudspeakers. Those are "simple" to use because engineers have already done the work for you. I'm not saying this is the only way to get improved sound from your existing system, but maximizing the results is a complex process!