Bxxxb,
I have heard the Concerto, but not in my own system, so it is very hard to compare to my Concentra. I did do some research before buying my Concentra, and (predictably) found differing opinions. The people at JRDG believe that the Concerto is a big step forward, but of course (with due respect) they need to sell the new version in order to make money. Some of those who have heard both (including a couple of dealers with whom I spoke) don't believe that the sonic differences are great, and a couple actually preferred the sound of the earlier (Concentra) models.
Two things are certain: the Concerto is smaller and runs cooler (using "Ice Power" chips) than the Concentra.
I, like most other Concentra owners, am so happy with the amp that I simply can't imagine there being enough of a sonic difference to justify the added expenditure.
Tony