Hello. In a way to go for deeper bass Extension, l just get rid of treble panels in the second set. (Easier to set, manage... and yes, l will brace them with a central beam, l can't manage rods to the wall...)
I found a discussion here (2008; sgr) with MG proposing a trapezoidal setting with 2 pairs of loudspeakers. I will happily go for it.
I much appreciate your considerations about the internal crossover! as a) EB21 is active on the bass end but passive on the hi end of spectrum; and b) power requirement is 80% or more dedicated to the bass section.
I just recapped mine.
Options remain open, whether to amplify the additional 4 mid-bass panels separately (4ohm) or in series with the existing ones (8ohm). And, at which cut-off (1000Hz, or less?)
Given the above considerations, and practicality , first option is simpler and better, l might say... so that the initial Tympani set is left as it is.
Initially, l was considering to add only 2 panels as sub-w below 100-150Hz, which means breaking panels and favouring only pumped up music vs musicality and ambiance...
TY for your interest. After 35 years, l smile every time Tympani get alive, their sound is effortless all the time.