To elaborate on my above post, accepting that the OP is not so inclined, but it may help someone out or at least be informative I hope.
I've constructed similar with good results, and as I alluded to, some commercial subs use a similar arrangement internally to provide high level inputs.
Because only resistors are used, this arrangement introduces no significant distortion to the signal, and because the impedance is relatively high (compared with a speaker load), it has no impact on amplifier performance.
These values will be good to about 100W into 8 ohms main speaker output while keeping the sub input to around 1.5V. it does this by using a resistor network to divide the voltage (at 100W, about 30v RMS) by approximately 21 (1.42v RMS).
This won't work if the amplifier is bridged (uncommon in home audio, generally), or for unfiltered LFE input, ie. the sub needs to have a low pass filter.
Note that car audio high level/low level adapters tend to use audio transformers, because bridged outputs are common in car audio especially head units. This isolates the outputs from one another so that nothing is shorted out via the common ground on the line outputs. Same applies if your amplifier is bridged, that's probably the easiest way to do it. But audio transformers, especially physically small ones, do introduce distortion. So if you can avoid them, all the better.
Anyway, here's a little how-to.
Materials:
4 speaker terminals
2 RCA jacks
enclosure to taste, shielding isn't necessary
OR
build it into a cable and use heatshrink
One end (speaker) bare ends
Other end RCA plugs
Long enough to reach from amp or speaker terminals to sub
2 x 20k 1/4 watt resistor
2 x 1k ohm 1/4 watt resistor
2 x 100 ohm 2 watt resistor
For each channel (ie. X 2)
* speaker+ - 20k - RCA out/center
* RCA out/center - 1k - RCA gnd/outer
* speaker- /gnd - 100ohm - RCA gnd/outer (will protect against shorts if you connect one channel wrong or if connected to bridged amp)
Connect RCA gnds together.
DO NOT connect speaker-/ gnds together.
(Note - you don't technically have to connect both speaker- terminals, as they are common anyway. In that case you can leave the 100 ohm resistors out and connect up the RCA ground directly to your amp ground / - terminal)
Testing:
Power off amp and sub.
Wire the speaker terminals in parallel with your main speakers and connect the RCA jacks to the sub with an interconnect.
Connect and start source. Electronic/drum and bass tracks work well here for obvious reasons.
Start with volume on amp at zero, balance center, and volume on sub at zero, set the LPF frequency to the highest for now.
Power on amp and sub. Wait for protection to click off.
Slowly increase main amp volume until the source is audible on main speakers. Make sure both channels are playing.
Then bring sub volume up slowly until sub is also audible.
Turn balance control all the way left. Confirm you still have sub output.
Repeat for right.
If no sub output at any of the balance positions, switch it all off and check wiring.
If it all works, set your levels up and enjoy!
