Welcome to TE!
I've never used the Rapha shorts, but--
If the soft-tissue issues are new with this pair of shorts, then it's the shorts, not the saddle. Shorts, like saddles, are really personal and even the super-fancy or universally-loved models won't work for everyone. How long have you been doing the 50-70 mile distances? You know how your sitbones had to toughen up when you started riding? This is the same thing. Your butt will be sore until you put in enough time in the saddle for it to get used to the idea. If you've been riding those distances regularly for months, then you might look at changing the saddle. The Oura is a racing saddle--very little padding. Have you considered something like the Jett or Ruby? There's a bit more padding in the Jett. If your LBS doesn't have it, they should be able to get it.