Passive inline high pass filters?

Does anyone know of a supplier of these? XLR or RCA

Do they degrade the signal path too badly?

Should I go with a sub with filter built in?

Please help...
These would most likely need to be made for you as function of the input impedance and the frequency you wish to cross at. Most likely this will sound better than using the sub's xover if you get it right.
with a little expertise you can fabricate your own first order filters. This is simply a good quality nonpolarized blocking capacitor soldered in between a male and female signal connector, with the shield wired straight through, or you can even solder the caps into the input signal path of a component, which avoids the sonic degradation of the two extra connectors that the adapter comprises. The additional connectors don't cause huge degradation or anything; just that less is more in this case. However, tuning experimentation is easier with the external adapter types, and then when you find the desired value of your capacitor you can then incorporate them into the component if desired. One cap for unbalanced; two caps for balanced. Use economy caps to determine the desired filter cap values, then perhaps get better quality caps for the final design.
Basically start out with something around 0.75uf caps. Because capacitive reactance varies inversely with frequency, the lower freq's which you want blocked see a higher (lossy) impedance across the capacacitor, and the higher freq's essentially see a short circuit, passing right through it.
Capacitive reactance Xc = 1 divided by 2 pi * f*C.

(C in farads) so 1uF is expressed as 0.000001
(f in Hertz)
(2 pi = 6.28)

Theoretically: at the frequency when Xc = the driven device's input impedance, that is the filter's -3dB point.
So for a 10k-ohm (10,000) input impedance, the -3dB point at 15Hz takes a 1uF capacitor value.

A 0.75uF cap with a 10K input Z yields a -3dB point of around 20Hz. The smaller the cap the higher the -3dB point. A 0.5uF cap yields 30Hz.
It's more easily determined in actuality with an audio oscillator and signal voltmeter while driving the actual equipment. However since you'll be tuning by ear anyway, lack of test equipment is not such a big impediment. Play around with this approach; it's certainly more cost effective and should allow you to eventually arrive at a more optimum result than purchased product. Try it out and have some fun, while learning something as well.

Or just buy some bass blockers from Scoshe, Crutchfield, or Audio Advisor. But unless you know the input impedance at which the -3dB point is spec'd, you're only approximating the -3dB filter frequency. So buy a number of values so then you can experiment with tuning by ear.