High pass filter need

What is the best way to add a high pass filter to my front mains (Dynaudio C-1s) from my Mc intosh pre to McCormack DNA-2 amp. 
A seperate line out from the pre goes to my JL sub which has its own filter. 
I want the cleanest option that will not alter the C1 sound other than remove the lowest octaves they can’t really handle that the sub can. id like to reduce the amp and speaker from trying to work on the low end. 
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. 



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.