DIY subwoofer high pass filter after preamp

Hoping to get some sage advice from some of the brilliant members of this audio forum. I am interested in trying a subwoofer in my system. I am not really interested in ultra powerful bass, but I am quite curious to see if taking some of the low frequency work away from my monitor speakers can further enhance clarity. I am also interested in improving the soundstage and filling out the music more.

Details of my system are in my profile but the speakers are Dynaudio Heritage Special monitors. I do not like the idea of using the subwoofer DSP for high pass filtering and would like to implement the simplest, highest quality solution without breaking the bank. After doing a little research it seems one simple passive approach would be to run the sub in parallel via one set of preamp line out connectors and then use a simple high pass capacitor introduced between the second set of preamp line outs and my amplifier.

I was thinking I could have a go at doing this myself. The plan would be to buy a high quality capacitor and solder one end of it directly to a male RCA plug I would then solder the other end of the capacitor to a female RCA plug and shrink wrap and jacket the whole thing together. I would then just have a filter/adaptor at one end of my interconnect that would connect directly to either the amp or preamp.

A few questions:

1) Is this feasible or even a good idea? 

2) What value capacitor would I need for roughly 80 Hz filtering with a 37 kOhm, 220 pF input impedance on my Audionet Amp1 V2? My speakers are 4 ohm, but not across all frequencies. 

3) Would I need matched capacitors for each channel and to within what % of each other?

4) Can I solder the capacitor leads directly to the connectors in this way? Most capacitors look to have enough wire for this. 

5) Anything else I should consider with this plan.

Thanks in advance for input.