Capaldi has been channelling Tom Baker since he took over the role.
He has a similar vocal range and timbre, and both actors have a theatre background, which means they're both trained to project, hence that booming quality. Peter has only to switch to a Received Pronunciation accent to complete the effect. He occasionally channels Sylvester McCoy as well, though nowhere near as often.
I'm surprised more people haven't picked up on this before, though perhaps it's because the music tends to be mixed far too high in most episodes. The music in this one was particularly well done, I thought. (The same director, Rachel Talalay, also worked on the final two-parter of the previous season, which also had a less ham-fisted sound mix than usual.)
Biting the hand that feeds IT
