What are the chances of failure if I drive a 34v device with 34.5v supply?
To expand on the comment from Colin-s, a simple silicon rectifier diode drops about 0.6V when forward biased.  This would drop your 34.5V down to 33.9V.  Just get one that's rated for the current you need.

As a handy side-effect, if you put it on the power input of the amplifier, it also protects against somebody connecting up the power supply the wrong way round.