Advice please on best way to wire these speakers?

Jack1991 wrote: Why im getting confused is because i read that a 4Ω speaker requires more power from an amplifier than an 8Ω speaker to produce the same loudness of sound. So now knowing that, if i parrallel bridge 2x 8Ω subs they will recieve more watts than if they if they are on there own channel each. But then as they are in parallel they will each be 4Ω so will require more power than an 8Ω speaker to produce the the same loudness of sound. But if they are on there own channel each they will recieve less watts but then they will remain 8Ω so will need less power to produce the same loudness as a 4Ω.

Same brand/type speakers of different impedances need exactly the same power to produce certain SPL.
A 4 Ohm speaker will draw twice the power compared to an 8 Ohm speaker. Half resistance means twice the power for a given voltage the amp puts out at a certain point. 2 Ohm speaker again twice the power compared to a 4 Ohm speaker and so on.
But there's a catch... amplifiers can't deliver power without limitations.
You can see this in the specs of the amp already.
Without knowing what amp you have, my educated guess would be :
4x   70W @ 8 Ohm  /  2x 140W @ 16 Ohm Bridged
4x 100W @ 4 Ohm  /  2x 200W @ 8 Ohm Bridged
4x 150W @ 2 Ohm  /  2x 300W @ 4 Ohm Bridged

If you want to/can use 2 tops and 2 subs in a mono configuration, you could put tops and subs in parallel respectively.
Each type (tops/subs) makes for 4 Ohm impedance or each couple gets 300W or 150W for each cabinet. That's the most you can get out of that particular amp. With a very important side note : can this amplifier work in 2 Ohm long-term ? Not every amplifier can be charged with 2 Ohm, certainly not long-term/reliable.
Bridging means combining 2 amplifiers to get double power in twice the impedance, so a bridge charged with 4 Ohm means each amp in that bridge "sees" half the impedance, which is 2 Ohm for a 4 Ohm bridge.
Any other combination will yield less power. Like every cabinet on it's own amp channel, will draw 4x 70W and that equals about half the SPL compared to the Bridge mode.
Whether that's enough SPL-wise, I can't say, but a 3dB difference (half the power) is not all that much in terms of SPL. Depending on the brand/type of amplifier, that difference might be less than 3dB, because specs aren't always 'true', especially the ones given for lower impedances.
Without knowing what kind of tops/subs you have, it might even be better to have not too much power on them anyway.

Okay, car amplifier...

Earplug wrote: When you bridge an amp, each channel "sees" the speaker impedance/2. So your 4 ohm load actually becomes 2 ohms. That is a very important limiting factor when thinking about using bridge on any amp.

Peter Jan wrote: You could see a bridge as connecting two amp channel in serie. Each half of the bridge (each amp channel) takes care of his half of the load., which is half of 4 Ohm = 2 Ohm. Total power of the 4 Ohm bridge is 300W and that's the same total power two amp channels put out when charged with 2 Ohm each. One amp 150W @ 2 Ohm + other amp 150W @ 2 Ohm = (no surprise) 300W @ 4 Ohm.