Here's a couple of example links. The first one includes the bucket and will never show tracks by Cleff. The second one has the bucket removed and will usually return more tracks including tracks by Cleff.
http://developer.echonest.com/api/v4/playlist/static?api_key=XXX&results=10&type=catalog&min_tempo=75&max_tempo=95&sort=tempo-asc&bucket=audio_summary&seed_catalog=CAZWSDF1498C4ADC80
http://developer.echonest.com/api/v4/playlist/static?api_key=XXX&results=10&type=catalog&min_tempo=75&max_tempo=95&sort=tempo-asc&seed_catalog=CAZWSDF1498C4ADC80