High pass filter

mode at 60Hz

Well the correct answer is a parametric EQ honestly. Even better with bass traps in the right location.  A single cap HP filter is an excellent idea when you are bi-amping but here it's going to just make your system sound too lean.

My suggestion is get an inexpensive miniDSP and try it.  Or if you stream with Roon it has DSP filtering built in. 

If you are just dying to solder something try an LC in series, I think 0.7mH and 100uF could work, I may have done the math wrong, with a variable resistor around 30k-100K pot around the whole thing so you can adjust the dip depth.  You wont get what you really need which is variable filter width but hey, worth playing with. 

A approximately 9 ft ceiling would produce a strong 60hz null at the 4.5 ft height.

Do you have a sub?

Put a sub on the floor by the front wall.

Get a 2nd sub, a in-wall sub like this sub from SVS or similar and put it on your ceiling by the back wall.

https://www.crutchfield.com/p_9463KIWDSS/SVS-3000-In-wall-Dual-Subwoofer-System.html

With correct phase adjustment, that null will be gone. You can't peq your way out of nulls. 

You will get rid of heightwise modes, lengthwise modes and widthwise modes in  your room by correctly placing just that one sub on the floor and one sub on the ceiling. 

@ohlala  wrote

My goal is reduce a vertical axial mode at 60Hz produced from my main speakers.  

Thank  you all for the thoughtful responses. I very well may try a first order filter and will looking to an LC circuit like Erik suggested. Those inexpensive RCA filter plugs would be a simple experiment. The Hsu could also be worth trying, also inexpensive. 

I have a pair of JL fathoms which do not excite that mode. The issue the main speakers. I have done hundreds of measurements and changes over the course of three different speaker pairs. The CR1 fixes the 60Hz anti-node issue. I sold it due to the distortion. A crossover will do the job, but I am trying to not pay the penalty for using one. 

In the past I raised a pair of much lighter REL subs halfway up the wall. It did help. It is a good idea, but I do not want to do raise a 130lb sub off the floor in my living room. I have acoustic treatment including multiple bass traps. For my own sanity I am not going reinvent that wheel ;). That was a long road. Thanks again

My room has the same issue. I solved it by using a Marchand 24db HPF at 80hz on the mains. My sub, sitting on the floor, takes over from there. I use an SVS 2000 PRO which has excellent DSP built in to tweak the low end as needed. If your room is large enough, 2 subs are needed. 

You should really just build the GR Research High Pass filters that go between your preamp and amp first before you start mucking around with more expensive/complicated solutions.  You might find that they actually provide exactly what you are looking for. I borrowed a JL CR1 and bought a Marchand and Sublime K235 and ended up using the GR Research filter combined with two SVS SB17 subs.  By rolling off the lows for the main speakers and then tweaking the subs with XO point, phase and PEQ I achieved what I think is exceptional sound.

https://gr-research.com/hi-pass-filters/

The GR Research filters are very easy to build, especially if you are using RCA connections.  I used XLR and that is more time consuming and you need to make sure the caps are closely matched.  Russian Paper in Oil capacitors are cheap and sound good.  Russian teflon are more expensive and sound better.  Auricaps do not sound that good.  I have some expensive teflon caps coming that I am looking forward to trying.

Also, that 60 Hz peak is likely a room mode so playing around with position and bass traps might help.