Lots of new things going on these days with aftermarket equipment. some new amps , such as a jl audio rd400 I have has the switchable option to be turned on by the speaker level signal only, rather than needing a remote switch input usually supplied when aftermarket stereo is switched on.
Just a low amperage signal is all that is needed for amp turn on. Could be a switch to a any switchable circuit Amp power is another story , especially with higher power stuff. Small amps take 8 ga wire direct to battery (fused near battery), bigger needs 4 ga, really bigger needs 1 ga. 2nd batteries & larger alternators end up being needed with more power.
Best control over system including high impedance conversion would be an eq , such as the 7 band Clarion I have . Has separate sub volume knob and it's own volume knob which gives you better sound control. You can adjust receiver volume & amp input for best noise control, the total volume with an actual knob. Can also use a combination of receiver and eq frequency adjustments. Has an auxiliary circuit also, which could have many uses. Could even hide an aftermarket receiver elsewhere. Most decent amps also high high & low pass options among others.
If you currently have no existing equipment, no need to buy a separate line convertor.
Please note that you will Never get the same distortion free clarity from speaker level inputs.
If you're not nuts like me right now, you can get a single 5 channel amp to power 4 speakers & a sub. If i remember right, it takes 10 times the power to double sound output, so your most cost effective gains come early. Maybe fleming can clarify on this.
Stereo is kinda like horsepower. You can get an affordable improvement all the way up to all out insane, dependant on budget, vehicle space , & time. All works together as a system also, matched system that is.. Can also get some parts a little better than others now upgrade as you go. Also, keep in mind that your ears gradually adjust to what you hear & really appreciate clarity..
With subs these days , 1 single can outdo 2 larger old basic subs, but box design is more critical now for best performance.
Check out my stand alone stereo thread in similar threads below. I need to update it, did a lot of experiments with it. Every vehicle interior has it 's challenges.
I still got an old aftermarket cassette receiver with dolby c, etc I can give you if you come get it in Fla..
I also have a Planet Audio sub amp, but Ben, I consider you a friend. Wouldn't give it to a friend. You can pm me on anything if you'd like. I've learned alot of little things past few months from a few, such as fleming, along with my own research , and painful trial & error. I'm being F-word picky on my work truck stereo right now & it is impressing many people on both volume & clarity, still being very in progress stage.
Also really just noticed you mainly asked about subs, not your full range speakers. You could just get a monoblock sub amp, most these days have the basics you'd need, minus wire & fuse to battery. I've heard Korean made amps, such as Skar or Sundown are pretty decent quality for the money. A Kicker amp I have from 2007' was made in China, so don't be fooled by most name brands in this day & age. Some of us may have given way more info than you need right now, but be advised that good stereo in a daily driver is as addicting as horsepower and good suspension.
Edited 3/6.
