You're basically combining EQ and phase shifts. I imagine as you keep layering on more it'll eventually sound very muddy, all boom and midrange and with weird overtones but I can also imagine somebody can find a creative use for it -- there are a lot of music genres out there so if it doesn't work for fusion jazz or Motown soul it could still find a home in, I dunno, shoegaze or maybe one of the more droney metal styles.
I used to spend hours experimenting with the Line6 DL4 echo pedal and found ways to push it into some very very strange sounds. Don't have any existing recordings from the time but I really wish I had, because it would take a lot of effort to recreate them. People tend to underestimate just how much range can be available using just one good stompbox.
Not saying your assumptions are wrong in general terms, which was what my question addressed.
But the tone I got out of it was very clear, well defined, articulate, punchy and snappy.
Definitely not muddy or undefined in any way, I hear no odd overtones either, and even though I do love a healthy dose of midrange, it doesn't seem to be overly emphasized, to the point where it gets hunky in any way.
I did try to run the cabs in parallel instead, which was my original intention, but it just lacked some of the punch and articulation running them in series got.
As said though quite a deal of parallel direct signal has been blended into each cab sim, so it we are not exactly talking 100% + 100% stack, even if I suppose the end result with parallel clean signal being added to the equation is even more stacked EQ's and phases,
My estimation from the individual IR frequency spectrums and the additional EQ added is that the end resulting EQ would be low end roll off starting around 70Hz or so, with a really steep roll off from 40Hz or so and down, and then a high end roll off starting around 5kHz or so, with a really steep roll off from 8kHz or so and up.
So if this estimation is right it wouldn't be overly mid emphasized.
Of course if the IR's had been, say, vintage 15" speaker cabs, and 100% mic/cab mixes, this likely would be the case.