Actually, you need 3 devices. Human brain is not optimized to find differences: if it is commanded to find a difference, it finds it even if there is no one! The brain is optimized to find similarities: this is how voice, speech, sound, also image, face etc. recognition works, which we constantly use.
So take three devices: 2x with one firmware and 1 with the other, and try to find the couple. Repeat it by flashing them randomly, by somebody else. I'm sure you'll be surprised by the result
I mean it: with
any firmware builds or mods on DX220!