Bob,

a lot of variables have changed apart from the issue of the shorts, since you are still getting used to a new bike, it appears? The shorts sound like a symptom.

The set-up and design of the two bikes, the frame material, the saddle, the relationship between bars and saddle, the tyre changes you've already mentioned, etc., etc., -may all differ markedly. Perhaps one of these changes may be affecting your riding comfort to the point that your backside is noticing the difference? ( the tyre remarks certainly hint at that). Your bike is bruising your body, the softest part that comes into contact with the machine. Even with better padding, a process of adaptation (bike and self!) may be necessary.

I agree with the earlier thought that weight is an overrated variable. It becomes the obsession, when attention to fit and comfort would pay real, and sometimes inexpensive dividends.

It's a bit like fault-finding with any other machine, changing variables, testing ideas, one at a time........ hope you find a solution.

Please ignore these ramblings if you've done a lot of trouble-free miles on the new bike, in road shorts...............