Theban Army, Battle of Luectra

[conon394]

Many Beotians would have then chalkidic corinthian or captured attic helmets.
the cavalry would favor the beotian helmet.
Demosthenes spoke more than 100 years after the battle and perhaps he was exagerating. Corinthian helmets were found in the tomp of the Plateans in Maratho

The Plateans were also buried with the freeded and enfranchised slaves so I don’t know that the arms found in their collective tomb can be seen as Boeotian per say. Yes Demosthenes (edit Apollodoros from Dem 59) was a century later but Mikon’s painting was only 20 or 30 years after the fact and the painted Stoa was one of Athens most famous public sites, I doubt he could exaggerate too much without looking ridiculous; the Stoa was not some obscure statute or point of diplomacy.

In particular I have a hard time believing that the Sacred Band would be using captured equipment, it members certainly had the resources to purchase new equipment.

Boeotian helmet was a cavalry's one, although many illustrators (even in serious publishing houses' books) used that helmet for infantry troops

Xenophon does indeed recommend it to cavalry, but I see no reason at all to say it was only a cavalry helmet. If the Boeotians liked it why not use it as an infantry helmet; with or without check pieces it seems to me it is essentially in the same category as a pilos.

So we would've seen both old(Corinthian/Attic) and new(Pilos) helmets?

I should be obvious that I doubt it. The Pilos was strongly associated with Sparta and the Peloponnesians; I doubt the Thebans would opt for that style give 4th century politics. Personally I see no reason to doubt that the best equipped Thebans would opt for the Boeotian helmet or the popular Thracian style helmets.