I know that setup - an English fellow had same setup -in which exhaust had a large plenum at 90 degrees to a restricted outlet from exhaust.
On the sliding can, the change was distance from header outlet to reflector end of the can. I tried a cone, a concave end and a flat plate as end of can.
The can stayed non resonant as long as the header was over a couple of inches from end.
power boost waspossible, when the reflector was aprox 1/2" from end of the 1" dia header.
My guess was that this determined the actual wave length in the header tube (also tried in various lengths) and the pressure in the can, provided the pressure for that wave to "self excite" if you will.
I have a classical training on the clarinet -as well as playing other reeds.
The musical instruments achieve their tones/overtones is a similar method to our tuned pipes.

On the clarinet -- the player sets the initial vibrations into the tube with a vibrating reed.
Then, the various holes in the tube , change overall tube length
Introducing the "octave key", changes the tube length responses.
what was an F ,in the open tube , jumps to a C above c ( f-gabcdefgab-c)
at this frequency, one closes some holes and the frequency jumps into another ,higher set of tones. To make this work, the reed on the mouthpiece which starts the vibes -must also resonate differently. The player learns to redirect the air into the mouthpiece.
On a trombone (slide type) there are a number of positions and depending on how the player forms the pressure into the mouthpiece, these positions of the tube (lengths) produce different frequencies.
On a bugle - the tube length is constant but the operator forms different pressures into the mouthpiece and you get quite a few different tones-each a mathimatical shift in frequency
On a guitar , a string is shortened and OR retensioned to produce a given tone.
On a 5 string banjo with Scruggs pegs, the operator changes string tension on one string whilst varying string length on the 4 parallel strings.
As you know - EVERYTHING is made up from vibrations- these are some of the simple ,audible ones. (string theory)
I knew when I started playing with tuned pipes that there were really no hard and fast rules ,such as the handbooks called out.
There are sooooo many dynamics at play--