I just played through the DB751 tonight. It sounds amazing and definitely different than the DB750. The sound felt more present, punchy, and upfront. The 750 always sounded a little 'squishy' to me (for lack of a better term), but I didn't notice anything like that in the 751 - It simply responded better. The tone controls are more flexible and produced a more noticeable change in sound.
I played a DB750 for two years so I'm very familiar with its sound. I think this amp sounds significantly better, in fact, I would venture to say it is the best amp I've ever played through.. Just my opinion of course.
Looks-wise, I agree with most of you in that the 750 looks a little nicer/more boutique. I'm not a fan of the newer control knobs and push-button switches. But, the cost-cutting strategy is understandable.