I think it depends on which unit you have. Mine calls for the 6350's but some of the others have the 12at7's. I accidentally swapped places (12au7/6350) and the gain was super low. Just make sure the tubes are what is called for and they are in the right place.
On my unit, the 12au7's are closest to the front.
On my unit, the 12au7's are closest to the front.