Stu31,
No problem. The 875 has higher grade components in it's chassis and is built even better than it's younger siblings, hence the £1k price.
The improvement in sound quality is immediately evident over the mid-range class. Some may lament that the 605/606/607 sound like they go louder - that was my exact thought when I first hooked it up too, but I soon realised that the way all that power is delivered is quite different.
While the explosive characterics of an Onkyo amp was evident in both, the 875 had a much more smoother and refined powerband, hence the more mature presentation.
In my view, the evolution from the 605 (1st generation HD amp) to the 607 (3rd generation HD amp) saw no improvement in pure sound quality, bar the extra features. The 605 was a tough act to beat, obviously!
The 875 (also a 1st generation HD amp), is a higher-end amp that handles steering effects with knife-like precision, and the bigger power delivery is smoother with greater control and grip over the sonic delivery. Detail resolution is up a notch too. The subsequent 876 (2nd generation HD amp) was unable to dethrone the 875 either, from a sound quality perspective.
It is also far more refined especially at the top end where the mid-range amps will struggle with harshness. While you may not be able to turn the volume all the way up on the 605/606/607, you can with the 875! It will be LOUD but composed, if you understand what I mean.
Finally, YES - in my view, the Onkyo TX-SR875 is a better amp than the Denon AVR-2310.
Ronald