Morton Feldman's "For Samuel Beckett", as performed by Klangforum Wien (on Kairos, 1999, in a rather spiffy cd pack with booklet) is just under fifty five minutes of the same off-chord, minimalist difficult listening. It can be regarded as "darker and heavier" in that it is certainly dark, but not in a deep, sonorous way. It has the quality of confusion, constant acceptance of current madness, that seems appropriate for a homage to the works of Beckett. Like faces that look constantly around for something to grasp onto and never finding it, but seeking anyway, always, forever.
I've always loved his incidental music for the Rothko chapel (New Albion Records, 1991, which comes with "Why Patterns?"), although admittedly it's much more structured and has a very lovely, uplifting melody in the last movement (that he apparently wrote when he was fifteen). It has wonderful soft, rushing passages and some amazing choir vocals, particularly part three which is basically a drone of female voices. But it's more for those seeking subtlety above darkness and heaviness.
By the way, I got both these albums for reasonable prices from the classical section of a big national record chain. It pays to pay a visit to such places from time to time. Because hardly anyone cares about this kind of music you can find good albums for good prices that will be untouched by the herd, yet happen to be there.