Send what, RE-400? They're quite dead right now. The driver's hair wires don't like standard soldering - tend to unsolder when the cable is changed.
Need to work really low temperature with these, just on the eutectic point of that Pb-free solder.
Of course, I might get another pair, but for now, getting more gear and RE-600 are on the shortlist.

Or maybe send the modded Beyer T70p. Since we know that headphone is so-so and I know exactly why. (See topic; plus the driver itself is overdampened mechanically, but that I can't really change.)

Oh, you mean HD800. The properly done mod from what I can see drops ~ 3-6 dB of treble. I think we have the measurements here already.
The problem is of course that the inter-headphone variance is on par with what the mod does. And the mod doesn't bring the mids more forward at all.