A base layer has to be matched with the activity level just like any succeeding layers. A 150 is fine for high activity levels, 200 for general use and the heaviest stuff for near arctic conditions of minimal or slow movement.
In the past I've used, and I know Randy Schwert uses fishnet base layers available from Brynje and Wiggys. This does mechanically give us that depth of microclimate Cody Lundgren discusses. Over that I like the woolen products.