I am going to disagree on the basis of practical experience. I've never had any success whatsoever removing ambience/echoes with a hi-pass filter.
As to the science… Yes, sound does bounce around in "hard" spaces. Yes, high end sounds diminish more rapidly as they lose energy more quickly. Yes, frequencies can change as they interact as they bounce around. But frequencies in the kHz range dropping to Hz range with volume levels significant enough to require a hi-pass filter? Nope, not in my experience.
My (very limited) understanding of the Kirchhoff Integral theorem is that the effect is more significant the higher the frequency. Light frequencies are far more energetic by several degrees of magnitude than sound waves, so the affect is close to unnoticeable at a sonic level in practical usage.
I've never heard of anyone solving ambience/echo issues with a hi-pass filter, but I could be wrong. You may want to bring this up on the JWSoundGroup production sound forum.
BTW, a man made tunnel IS linear when compared to a cave.
I didn't state the problem was solvable with a high pass or low cut filter, I said (and I quote myself) "it will help."
And just to re-affirm (I think you're saying the same thing as me), frequencies cannot change through reflection per se as everything we know about physics would simple be wrong. Rather they change when reflected by a non-linear, denser medium and / or a moving, denser medium (the latter you would know simply as the Doppler effect). And service tunnels would fit the first, part of this description unless the service tunnel is simply a perfectly, flat wall without ceiling or protrusions.
So, a high pass or low cut filter will remove a little bit of the echo and help. The amount of help cannot be easily calculated (if you read the research paper I provided, the physics / maths is in there) as it depends on the how non-linear the environment is.
My principle is that if I do ten things, each of which removes a tenth of the problem, at the end I have no more problem. This is simply one of them.
But if you love sound, try that cave. It's an amazing hypothesis that early man was storytelling through pictures and sound and if correct, would make that cave the first, ever Foley studio. I wonder if there was a producer saying, don't worry if you mess up, we'll be able to fix it in post...