the smoothing capacitors supply the transient current.
The rectifiers top up the smoothing caps.
I have power amps that use 1n5402 (3A) as discrete bridge rectifiers and they have never blown.
If you are using your amp properly then each channel could be ticking over with an average output of 1W/channel. Add on the quiescent dissipation of around 20W and total loading is under 30W.
That amounts to 375mA average rms charging current.
But rectifiers do not charge at RMS rate they switch on and off at mains frequency rate and peak currents are of the order of 10 times the average rate.
the peak current of many diodes is around 50 to 200 times their continuous rating.
Thanks for the replies, guys - I have replaced it with ultrafast, soft recovery diodes as the rectifier & notice a slightly smoother, more silky sound - I just wondered if there was enough juice in the bridge now but it seems OK