Gain and linestage

If someone could, please explain how a preamp can impede the rated power of an amplifier?  At least that's how I interpret the discussion.This was a discussion I was reading on the Butler TBD 2250 amp and someone looking for a good matching preamp.Hopefully the link comes through below.
https://forum.audiogon.com/discussions/preamp-recommendations-for-a-butler-tbd2250?highlight=butler
Thanks