rcas vs speaker level input vs locs

Hi, I'm going to be getting a Mazda 3 soon which I intend to add a couple of amps to using the stock head unit. I'm thinking of adding a 400w (RMS) mono amp to drive a 12" sub plus a small 2 channel amp (probably ~300w RMS) to drive the front 6.5" components, so nothing too fancy. Only problem is I'll have to use either the speaker level inputs to directly drive the amps or buy an LOC to give me a few RCA outputs. Having only ever used RCAs from aftermarket head units, I've no experience on the sound quality from high level sources. Can anyone share their experiences, please? The amps I'm thinking of using are from the JBL GTO range which have full range preamp outputs, so I'll only have to connect the HU to one amp, then daisy-chain the other to it. Thanks

Hey first of all to get truly good sound from a factory headunit you need a equalization corrector like the jl audio cleansweep or the new re q product that I think is coming out. The reason for this is that the factory headunit is set to lower the bass the higher you turn the volume and this among other things is not something you want. For regular people it may not matter that much because a good amp and sub will still bump. A loc is needed when you have a factory amplified system which means the signal its putting out is not what the speaker level inputs on your aftermarket amp would like to see. The loc allows you to adjust the level. I think the higher end MTX amps eliminate the need for line output converter in all cases cause I think they have adjustable speaker level inputs. I would check into that. If they sell a dash kit for your car then just get a new headunit for the ultimate control over your sound.