Kingharold asked, "would four horns in the four corners achieve enough improvement to be worthwhile?"
Todd Welti of Harmon International investigated symmetrical subwoofer placements using simulations, and four subs with one in each corner was one configuration he examined in this paper: https://www.harman.com/documents/multsubs_0.pdf
Corner placement optimizes for low-end extension, but not for smoothness.
From the paper: "One subwoofer in each corner has good low frequency support, but does not perform quite as well as one subwoofer at each wall midpoint..."
So imo four subs, one in each corner, is likely to be an improvement over two subs in two corners. Whether or not the improvement will be "worthwhile" is a judgment call, but I’ll go out on a limb and say "probably". The reason I think so is, the ear has a heightened sensitivity to frequency response issues in the bass region. This is implied by equal-loudness curves, which bunch up south of 100 Hz, such that a 5 dB difference at 40 Hz is subjectively comparable to a 10 dB difference at 1 kHz. In English, a little bit of improvement goes a long ways in the bass region, in my opinion.
Also, the Fitzmaurice Tubas are big enough that you could probably orient the mouth of each one differently with respect to its corner and get a little bit of asymmetry that way. And if such orientation is not an improvement, easy to undo.
Heaudio wrote: " a specific version of a bass array, also called DBA, but in this case double bass array, may be something to look at..."
My understanding is that a double bass array normally consists of four subwoofers on the front wall, each 1/4 of the way in from its nearest corner (this configuration theoretically producing a planar wave). Then a second identical array is placed on the rear wall. The polarity of the signal going to the rear array is inverted and it is delayed by the amount of time it takes for the sound from the front array to reach the rear wall, the intention being to cancel the front wave when it reaches the rear wall.
(Personally I try to use the term "distributed multi-sub system" rather than "distributed bass array" because the latter invites the abbreviation "DBA", which as you pointed out also stands for "double bass array". Not that I object to anyone else using "distributed bass array" abbreviated as "DBA", as the meaning is usually clear from the context.)