Just goes to show how powerful expectation bias is, 2M will have absolutely no discernable effect at the impedance levels of speakers (around 10 ohms). You are simply imagining that 2M sounded better (*). My vote is its a 20 ohms pad resistor to reduce the tweeter sensitivity (a 2 ohm resistor would be less likely to fry at sensible HF current levels, and would only be a tiny correction anyway).
To fry a 2W 2M resistor takes 2000Vrms and above, note.
(*) Unless you are able to do blind AB testing any auditory difference that's subtle is easily masked by expectation bias.