what about a little pinch of jack Russell? here are some lab/jack russell mixes;
(although your dogs seem stockier)
btw... here's a lab/basset hound mix;
more i look at the photos the more i dont think lab/ russell mix because your pups are a lot stockier. i'll leave up the photos though for you guys to have a look at. as i said maybe a bit of russell and multiple other breeds.