Andris Nelsons and the Boston Symphony are in fine form here, satisfying guides, as always, in their approach to the ironies and tragedies of the Shostakovich symphonies. Indeed, now that we know him well in Boston, it has become clear Nelsons is consistent there in the way he approaches music of this kind. But he illustrates, you might say, along with special romantic insights, the sins of his virtues. Nelsons is what Sir Thomas Beecham would have called a “ritardando” conductor. One notices this not so much in tempo variance as in the tendency to prepare for and draw out a cadence. Nelsons is not slow. But one is nearly always aware of a certain smoothness in transitions from phrase to phrase and a roundedness in the brass sonority he encourages from the BSO.