Beach220

Well I being new here I'll try my best.
I have the 2008 IS250 non ML stereo I wanted to get a unit with USB & Aux dont care about touch screen. I'm an ole stereo guy used to have a NAK 700 wish NAK would do better now a days. Anyway I bought a Kenwood 794 head unit I was told I cant replace the amp because its connected to the Air conditioning system. I bought a Scosche ITCLS01B to replace the face of the installed unit. It has a touch screen air conditioning system &
space for a double din stereo. It comes with its own connection to work with the original system. So not to spend a fortune 1) do I need to replace the speakers there working great 2) were are the crossovers are in the amp or do they exist in the doors. I would like to buy a new amp & bypass the installed amp for the speakers is there an adapter to add to the head unit & still use the old amp. Lately the radio goes on but I hear a pop from the amp in the back no sound. It has a mind of its own it I get audio from the (Radio CD) when it wants to so its a constant turn it off & on until I hear audio. I dont mind getting a new updated amp but its a matter of expense .
Its just what to do with the speakers. I'm not doing this work myself I have an installer but he's not to sure about the speaker thing. He has said there might a 2ohm speakers set up but I see photos of the speakers stamped
4ohm. Or can I just buy an aftermarket crossover to hook into theses speakers replacing the crossover from the current installed amp. Thanks anything from anyone would help .