Just a couple of points, food for thought only:
1. D&M switched to 4 ohm compatible not by boosting their power supplies, but by providing options to limit the output voltages. If you follow their instructions and limit the output V, you will not get more output into 4 ohms, may even get less. They also must have improved their protective circuity so they became more confident in not ending up with more warranty claims.
2. Regarding the 2008 (edit: 2006-7?) models, something seemed to have happened to that year's models. If you compare the weight of the SR7008 and SR7009 you will know what I am getting at. I am not one who would pay too much attention to weights between comparable models of different brands, but the SR7008 and 7009 are presumable designed by the same people. Both are class AB, same number of channels, and just one year apart, so it is reasonable to expect they weigh the same or almost the same.
Edit: I have the model year mixed up a little, the SR7008 looked fine, it's the SR7007 that was put on diet for some reason. The SR5003 actually is heavier than the SR7007. The 5000 series appeared to have been put on diet a year or two earlier (from SR5006 and later models).
The lighter weight could also be due to the use of lighter material such as plastic, Aluminum, and transformers and capacitors that have higher output to weight ratio, still I am suspicious about the SR7007 in particular.