Adding a subwoofer can add to new sensation of frequencies factory drivers don't hit. I went with 2 - ten inch jl audio 10w0v3. While back in the 1990s the W0 I didn't pay much attention to, running a JL 18w6. I enjoy the new w0v3 series. The new 10w6 is what I would go with if money were of no issue.
I like 10" woofers a lot. 8" woofers are great for mid-bass. 12" subwoofers are loud. 15" and 18" subs hit hard low and lower. While 10" subs to me sound pretty good for their size and they are easier to blend and smaller than 12" woofers. I also like their tonality more than 12" woofers, and would likely choose only a much larger subwoofer if 10" wasn't useful (e.g. PA). In those cases I like sealed 18" woofers. They hit low and deep while retaining accuracy. Anyway back to the 10" woofers, they sound great in the car, and I could potentially see pairing them with 8" woofers too. But I would go with dual 10" woofers and I did.
While the CLS C218, similar to W212 stereo isn't bad for a starting base, I added dynamat and upgraded the once I upgraded the front drivers things really got going, later I installed crossovers, all of this of course was car audio foreplay, art.
There are a few things to note. One if you have harman kardon with the subwoofer amp n40/9, I tap that socket and leave it unplugged. I opted to use two different tap styles to test both. The first are long traditional ones, you fit two uncut wires and the newer taps into just one wire and uses a spade connector for the second. This spade connector style is shorter and the connection seems sturdier as the spade presents a structural resistance, so I would use this one.
I found two blacks. One of which was wired into remote power on of the new amp. The second two wires are for the RCA style signals. White is positive and and I believe brown is negative but please reference the pictures.
I then ran a small run of wire from the small battery in the back, for the power on. I installed two speaker cables out for a run to two 10w0v3 which are rated for approximately 330 watts. I used my old hifonics hercules amp to provide 2 ohm outs for each sub a total of 750watts RMS. I found in this setup about 9pm on the dial worked well in 2 channel mode to provide a smooth listening experience, and in 4 channel and logic mode to produce a party like experience with the woofer being fed both the 60 hz low pass and the 80 hz rear low pass signal.
At 11pm I found two channel in flat mode to provide a great but slightly overpowering experience. Maybe 10pm is a happy medium? Moving foward and summing up this original post, yes the old forum posts work.
Thanks a lot to all the mbworld members who helped make this work, posts like: https://mbworld.org/forums/cls-coupe-c218/749105-adding-subwoofer-amp.html
Read how I mounted the amp and routed the wires using MB Cable management.