[ATR92]

You will need to do like Ja'Crispy said. Get yourself a mono sub amplifier that accepts speaker level inputs. You basically tap the existing speaker outputs instead of using RCAs.

OR,

A more quality option would be grabbing an AudioControl LC2i. It will use a set of speakers as input (rears or front woofers) and convert them to line level with RCA outputs. It also has Accubass which corrects bass rolloff that the OEM system may produce.

Have one in my 2014 Elantra Limited with Nav. I since have upgraded to the LC7i and am powering a sub amp, and a 4 channel with all new speakers. Works flawlessly and should work just fine for the new ES.

(Side note: I don't own an ES yet. Just a hardcore lurker looking to upgrade in the next few months and thought I could help. Audio is my strong-point with modding.)

 

[ATR92]

I want to convert signal to preamp but I want to keep all current speakers working as they are now, not off a new amp.

LC2i would be your absolute best bet in my opinion if all you are looking to do is add a sub. They can be had fairly cheap on Amazon/eBay and would produce much better quality signal than a traditional line out converter. Your factory speakers will operate as normal while the LC2i generates a signal for the subwoofer amplifier.

 

[ATR92]

Side note about the factory system and adding in a sub. I will say that the factory DSP settings of the headunit for the base model sport, suck. I haven't done a frequency analysis yet, but I can easily tell they have a steep roll off high-pass filter at 40/50Hz. This moves higher as the volume climbs, so set your input gain higher on your amp and tune your headunit for max gain at no more than 32/33. It does not play well if you are looking for good capture of sub-harmonic frequencies. Sure, it will bump, but if you're after SQ - an after market head unit is a must. I haven't upgraded mine yet, only because I haven't found one with a GUI I like. I am not sure why aftermarket touch screen manufacturers always have such terrible user interfaces. Everything is touch screen these days, why does car audio/video still look like it was built in the 80s? Lol... I love the UI on the stock system, just hate the DSP settings.

I can only imagine what nonsense DSP is built into the factory system. I know my 14' Limited with the amplified system starts to clip at a surprisingly low volume. Not much bass rolloff or distortion, however it IS present. The non-amplified systems are only worse. Ja'Crispy you should check out Kenwood's new line of double din units. I just installed the DMX7704 into a co-workers car and was blown away. Extremely responsive unit, nice UI, DSP/Time Alignment and much more built in I couldn't believe it.


OP, if you are following basically as you turn your volume up there will be a point where the bass no longer increases and may drop out. That is called bass roll-off. Using a traditional line converter will work, but the bass roll-off will still be present. Imagine your new sub hitting nice, then turning the volume up to lets say 20 (hypothetically), and all speakers get louder except for the sub. An active line converter like the LC2i will pull that bass back into the equation and even boost the frequency if you set it that way. I just feel you will be much happier knowing you got your moneys worth VS wondering why your new expensive setup isn't living up to your expectations. Just my $.02