Speaking as someone who has regularly (14 times on my GS) done 1000 mile days and who has tried quite a number of combinations - the best is LD Comfort shorts, but if you find, as I do since I broke my femur, that the elastic around the leg is uncomfortable then the next best I have found are shorts made from bamboo. Whatever, you need to look for underwear without seams which is what will give you pain after a while. Socks - silk and wool (not merino).
As for merino wool - just seeing Pink's video on Peta's website (
https://secure.peta.org/site/Advocacy?cmd=display&page=UserAction&id=823) was enough to make me want to boycott it.