One word: "wide". Buy the widest tires that will fit your bike.
The XR is out of production, which is why you may be having trouble finding it (although it is still being sold in limited quantities online by places like wallbike.com). The Dureme, Supreme and Extreme are all reasonable replacements for the XR. The Dureme is the official replacement for the XR. Read the descriptions of these and other tires in the Schwalbe Marathon series to see what most closely matches your situation. But don't fall to analysis paralysis--any of these will do just fine. For rough/non-existent pavement, the Extreme may be your best bet.
Since relatively few shops in the US keep Schwalbe Marathon tires in stock, you may need to special order them or order them online.