Saying the the double rubber diaphragm rarely goes bad is a surprising statement. It is in fact usually the small round rubber diaphragm in these that fails. The symptom them is low <1.5BAR control pressure even when warm. Most aftermarket suppliers of diaphragm kits don't offer the double diaphragm units because they are technically difficult to manufacture.
The double rubber diaphragm wurs are far more prone to problems than the simpler single rubber diaphragm units in 4.5, 6.9 etc
What I meant by this (which, my bad for not being more precise, too many diaphragms, not enough explanation) is that the guy from Salvox said that the lower rubber/steel diaphragms are very rarely problematic, it's almost always the upper round diaphragm which goes bad. That's why they don't bother with a kit that replaces the lower one. I did find such a kit for $125, and will contemplate pursuing it if I'm still having issues after replacing the upper.
That's all on hold, at the moment, though, since I've a C220 to get back to being road worthy, and the starter on my 6.9 caught recently, so I need to get it on the lift and replace that, then worry about the WUR.