I concur with Myf. Amstaff/pitbull.
Look past the colours to the shapes of heads and ears in these 12 week old pups:
One of the biggest problems with id of bull breeds and crosses is that so few people actually know what a real pitbull looks like.
And don't listen to the mastiff x line. Most pounds, shelters and rescues call anything tan or red with a black mask a mastiff x. If they actually ever met a real mastiff they'd probably think very differently.
Tan and red with a black mask is a very common colour in SBT and APBT and their crosses.