I think that is how the XR45 operates normally. Ask Wayne at Bolder though for a confirmation of that.
Not quite. All the Panny receivers as far as I can tell (yes, even the 25) use a hybrid approach, where part of the volume control is done by altering the power supply voltage, and part is done by digital-domain attenuation. It sounds like 'HQ mode' is likely just eliminating the digital-domain component.
The only XR version I've measured in detail is the 25 and it uses 12dB of power-supply voltage adjustment, with a lower limit of about 8-10V. I *believe* that the XR10 was reported to go as low as 2-3V, though, and I'm not sure about the 45 or the newer receivers. The upshot though is that eliminating the digital domain component will introduce a 'maximal attenuation' limit, and depending on your speakers this might be too loud to be practical.