I could be way off as I’m no expert on the nuances of the transformers but I think the raving is about the simulclass in general whether switched to Class A or Simul. The tone that poweramp provides is different to a 60, 60/100 etc irrespective of the setting.
I haven’t heard an SRG or HRG in person but if using the JP as a proxy for the HRG (assuming they sound close) I find the DRG to be squishier and sweeter. A touch less abrasive and I personally prefer the DRG to 60/100 based purely on this comparison.
Between the two switches on the DRG I personally find the DRG switch adds volume and power potentially more punch in the low end. Similar to the difference between triode and pentode. That’s what I hear anyway. I generally have mine in Class A but switch every now and then.
It would also depend on what tubes you have in the outer sockets. With the DRG’s option to run 34s on the outer pair it changes the tonal characteristic as a whole, again irrespective of Class A or simulclass setting. The differences between DRG and HRG might be less pronounced if running all 6L6s in both amps. I run winged C 34s in my outer set and I love what that brings to the amp.
That said, each have their own favourites. Some rave about the HRG, some about the export transformers, some of Coliseum, Doug West - the non GEQ etc etc. There’d be variability between each subset as well.