High pass filter need

You are going to laugh. 

Seal the port with a t-shirt or old socks. 

This will raise the -f3 point, as well as reduce excursion below the port tuning frequency. This will not however reduce the amplifier load. 

To reduce the amplifier load you need a line level crossover. What I suggest is you actually listen to the filter in the JL sub first and see if you hear any negative effects (besides less bass). 

I still recommend you seal the C-1s though. That will make integration better and give you more dynamic range. 

Best,

E

@bundy  

I'd suggest the Harrison Labs FMOD inline high pass RCA filters. 

I've used them, and they are quite transparent, and do the job just fine.  They filter at 12db/octave, so whatever value you get, take half of that value, and that becomes your -12db point.  For example, I have a pair of the 50Hz high pass filters, so the freq response is -12db (attenuation) at 25Hz.

They are around $25 ea at Amazon, Parts Express, etc.

I’m selling a couple of passive high pass filters. Marchand manufactured. 

Thanks for the suggestions. The JL sub doesn’t have high pass outs so not an option. The Harrison labs FMOD look easy enough to try. Heck, I work in Dayton. (Location of Parts Express)

i am more worried about over driving the C1 woofers than the DNA-2. Its a beast. 600 w per ch into 4ohms. So the port stuffing is also worth trying. 

You should check out this HSU high pass filter!
http://www.hsuresearch.com/products/high-pass-filter.html

To block highs from your main amp, at a specific chosen frequency, you have to know the amp’s input impedance, and choose the correct value of capacitor, to install in series with the input signal. The old Dahlquist DQ-LP1 passive/active crossovers used this method, in their high-pass section. In the Dahlquist manual(I’m providing the link), you’ll find a table(and formulas) that will help you determine what value of cap(and perhaps, resistor) is necessary, to meet your desired crossover point. Dahlquist used a 6dB/Oct slope, because higher orders cause phase shifts. These typically very small value/size components, can be easily installed either inside your preamp(at the outputs) or inside the amp(at the inputs). https://sites.google.com/site/mpbarney/home/dahlquist-dq-lp1 

If you are running your system balanced this works well:
Ashly XR-1001
https://www.amazon.com/Ashly-XR-1001-Crossover-Connection-Chassis/dp/B001UIQ1CM
I used one for a while before upgrading to the JL CR1 which has a slightly better feature set but at more than 10X the cost.

BTW, the Ashly has a jumper that you can remove to bypass the 20Hz hi pass (rumble) filter.