We Hamburg people are pretty much biased about our new music house in town: Elbphilharmonie. But I love it, as I do most music venues. I come here quite often, as the ticket prices start as low as 15€ and I have always been fortunate enough to get one or two during the seasonal drawings when they reopen online booking.
The thing is: if you are in town and flexible, I recommend checking out the booth nearby the Elphi, where quite often the last seats can be purchased on the same day, as the big cruise liners return the packages which they haven’t sold to their guests. Sometimes there are guest performances by e.g. Kampnagel who have their own tickets as well. On their website they provide an overview of available seats as well.
And always check the venue! Under the umbrella of Elbphilharmonie there is Laeiszhalle as well (awesome place!!! Old school! But not Elphi). That said, and if you didn’t get a ticket: just go and see the view from the plaza. It is awesome by itself. Especially during sunset. And come back. Maybe next time will be your time to grab a seat.