While much of the focus was on the Doctor's guitar, it was his amp that really provided the subtle nod to the past few years of the show. The make is Magpie Electricals, which has appeared numerous times since the show's revival. It's the name of an electrical shop in The Idiot's Lantern, the make of a television, the company who make parts of the TARDIS during Matt Smith's time, and another electrical store (only this time in the 33rd century) aboard Starship UK. Also, notice the squirrel atop the amp? The Doctor says that he took apart the radio in the TARDIS in order to make a clockwork squirrel, and here's the proof of that actually happening.