[Chaos]

It is entirely possible to adapt some OEM speaker level outputs directly into the RCA inputs on some aftermarket amplifiers. It depends on how much voltage the OEM source provides, and the input sensitivity range of the aftermarket amp.

Heck, just one example of this process is printed right in JL Audio manuals, but still nobody seems to believe this.

 

[Chaos]

Amplifiers inputs are voltage sensing, not current sensing. This is why you can use Y cables almost indefinitely on the inputs.

An LOC is an easy $50 for a shop, so of COURSE you need one.

At least somebody gets it

If you wire speaker level outs to an amp that does not support high level it usually has a very loud pop when turned on and sounds like the gains are wAy too high, basically sou ds like crap. It can damage the amp, all amps are different but that's been my experience with such attempts in the past...just get a LOC and be done with it.
With high level inputs on amps, I think it's better not to use them, usually get better SN and less distortion with a quality LOC

Yes & No. Like I said, it really depends on the specific equipment you are trying to integrate, and where you are tapping into the OEM signal. Sometimes the end result sounds better one way than the other, but I don't think I've ever seen an amp actually be damaged by high-level inputs.