Sore bum....

Probably best just to enjoy the discomfort. Whoever said us cyclists have a right to be comfortable?

I'd check that it's the right width - there's videos online to measure your sit bones. It sounds like your sit bones aren't hitting the right area on the saddle. Ideally, do the chalk/cardboard or foil method, and line it up with your saddle. 

There's a chance due to insufficient torqueing that your saddle may have slipped back, or your bars down, which could also change your weight distribution (though the latter usually manifests in upper body problems).

Also check with a spirit level that the padding hasn't slipped on the saddle - there was a problem with some women's saddles a couple of years back with one side lower than the other. 

I was having sit bone soreness all of a sudden and wondered about changing my saddle. I discovered one day it was actually as a result of carrying my wallet in my back pocket and therefor sitting on it and nothing to do with being on the bike at all.

I think some blokes, particularly bean-pole types, suffer from achey arse syndrome more than other people do. I've just ordered a Brooks B17 in my search for armchair comfort. I can't wait to see how weird it looks on my full carbon Giant .

Exactly the same situation for me.

I have a Fizik Antares saddle which I find very comfortable for shorter rides but anything 50+ miles & my backside gets sorer & sorer the further I go. Do the usual moving about & getting out the saddle & stretching every 15 mins or so but still have the same problem. Towards the end of a decent ride I'm in & out of the saddle like jack in a box.

Don't think your shorts are the issue TBH, I use mostly Castelli & they have a decent enough pad.

I remember seeing Alex Dowsett on a GCN video saying that the best piece of advice he'd ever been given was stop trying to find a comfortable saddle just find one that's more comfortable than the rest you try. "It's a saddle not an armchair"!

That said I think I'm gonna see if I can get an Antares VS or VSX with a relief channel off eBay & give that a go.

I'd certainly be trying a different saddle if you haven't already if I were you.

Actual aching in the cheeks or are you just being coy?

Any other set up changes- saddle height/angle etc etc?

I've had a rubbish winter/spring with tail end issues after switching to my previously comfy old bike and being slow to spot that the saddle was slightly off centre.  That set up a vicious cycle of sores that has taken a while to settle down.