Before I start there are so many options other than what I'm going to suggest so please make alternative suggestions. My assumption is you have an existing stock HU (low power) but you also want to use your phone wirelessly to play music. You want little more volume than the stock HU and you don't want to have to buy a new alternator. Understand even if you have a 1000 watt amplifier it will work with the stock alternator it just wont get that loud without trying rob power from lights, battery recharge etc. So we need an efficient amp and high sensitivity speakers that can play louder on less wattage. And all of this shouldn't cost a lot or take up too much space. And while you dont need an old school EQ you asked about it so that's a part of this solution.
I owned this
Clarion EQ in another car and had this setup - more or less. Put this under you dash. Connect you the speaker wires from your stock headunit to the Hi-level inputs on this eq. This will be your main input. Then use this
Bluetooth receiver and mount it behind/under you dash and connect the 2 RCAs to the AUX IN rca inputs of the EQ. This will be you aux music source which is your phone. Then use this small (4"X6")
MB Quart amp that should also fit under your dash. You will need 2 sets of RCAs - Front and rear - that will connect from the EQ to the amplifier. From your amplifier you will connect speaker wires to the front speakers.
This Infinity reference 3.5 are good. You will need to cut a piece of paneling do adapt these 2 speakers to the single dash speaker hole. I've used them throughout the years. For the rear speakers try
these 6X9s. When you turn on you stock headunit the EQ will turn on automatically and so will the amp. You can adjust the high and lows with the EQ knobs and can use the fader to balance front to rear. With the settings on the amp you can limit the bass getting to your dash speakers that they were not designed to play. For the Bluetooth you will pair your phone with the Bluetooth receiver ahead of time and when you start up the car the next time it will connect. To play phone music you press the Aux button on the EQ to switch to the BT receiver source. To control the volume you can use your phone or the volume knob on the EQ. And if you wanted a sub you could get a second amp and connect and control that too to the EQ.
And if you want a more expensive amp that is reviewed well and has a brand name try
this kenwood
You can go cheaper than what I described and for example if you don't care about getting music from your stock HU there are other options that make more sense. But this combo will give good sound at a modest price and allow both phone and stock HU to function. The EQ does double duty. It lets you change the HI-Lows and combines the 2 sources. The amp should have its own wire to the battery - or starter stud, maybe starter relay. I used to do more of this but newer cars are pretty complicated. I'm looking forward to doing my fury and will replace the EQ with something more flexible, add 4 dash speakers, kick panel speakers (PO already cut holes) and 2 amps - bigger than the one I suggested. Have a good rest of your weekend all. mike