Surely some people in this thread are "kidding"... Not knowing what a DX contact is, or how to "define it", etc... And people that never use an amplifier... I know it's not necessary in all circumstances, if you are high enough you can receive a distant station, then likely you can talk to them without an amplifier, but depends on how far exactly they are, and what their specific noise level at that time is.... 2 people can hold a conversation (QSO) say, for example, 100 miles apart, with low noise level, and only legal power, if antenna's/receivers are good on each end... But what if the noise level is up on one end or both, it happens, and you need something extra to continue the QSO more S-units to overcome the increasing noise levels... My powerlines do this from time to time dependent on temperature outside, and wind levels, humidity levels etc....
As far as making DX contacts with low power, yes, it's easy, when noise level and conditions are good for it. I hope using a "linear" wasn't illegal when I was doing CDX contests back in the day, because I was running 1200PEP on SSB via my M103, which added even a little more gain. Rarely could I not make a contact if I really wanted too...