Here is some reading :
I had a Class G Hitachi SR-804 receiver I bought in 1978 for years. It was a good but not outstanding sounding piece. Not familiar with the Arcam though.
Hadn't heard about Class G in years.
Class G enables higher peak power delivery in a smaller package compared to Class A/B is my understanding.
Newer Class D amps are the more popular amp technology it seems these days for those looking for more bang in a smaller package along with lower acquisition costs and power consumption benefits.
Often class G would be used in portable headphone amps. On rechargeable lithium battery these little ones can last upto 30 hours of continuous listening. It has advantages over A/B class almost on every aspect you can ever think of including clarity, noise floor and certainly efficiency. Call it improved class D.
"Call it improved class D"
Or is Class D improved Class G?
Class G has been around longer but never seemed to break out in home audio.
I suspect class D is more efficient. Also its no comparison soundwise between the Class G Hitachi receiver and current Class D Bel Canto ref1000ms I have heard. The Hitachi was not even close to being in the same league, though comparing a 30 year old 50 watt/ch (100 w/ch Class G) receiver with recent technology 500w/ch mono blocks is certainly not an apples/apples comparison.
maybe some others who have experienced both Class D and G can comment.
My gut feel is that Class G is a simpler technology for a Class A/B amp maker to implement for efficiency purposes than Class D, which a totally different beast from a design and technology perspective. Outside of that scenario, making a more efficient amp using more traditional amp technology, I suspect that Class D has the bigger upside in terms of its value for producing better sound quality in a smaller package that is also most energy efficient.
My understanding is that a Class G amp will distort differently than other amp designs, and that historically, distortion and with Class G how it exhibits itself in regards to sound quality (always near and dear to audiophile types)has been problematic. One would expect that newer Class G implementations would have a handle on that at least to the extent of providing competitive sound quality though I am pressed to see how Class G alone would result in better sound quality than Class A/B. Perhaps, this is possible at a particular price point, but not categorically?
I suspect Class D is a totally different beast in regards to sound quality and how it distorts compared to Class A/B/G amps so I am not sure a comparison is even warranted there.