I have to agree with SMD - have had DAB in both my cars for a few years now (only for PlanetRock), but recently the sound quality has gone downhill. It used to be 128 kbps in stereo which wasn't brilliant but at least it was listenable, but is now broadcast at 80 kbps in mono It's like being back in the dark ages.
Through a radio in the kitchen it's okay, but through a decent stereo system it sounds awful (got a proper DAB tuner on my home hi-fi as well and it sounds crap now - I use the Sky channel instead). You're better buying a Pure Highway and connecting that through the FM modulator, then you can listen to it in any car and use it as a personal radio as well (and it's cheaper). An external aerial will help the signal, but won't improve the sound quality.