10 watts RMS is sufficient for general music with a low to medium volume.
My subwoofer on my office speaker system is 25w RMS (true - not made up) and is pretty loud and clear. The subwoofer/amp on my home cinema is 500w RMS (possible but I feel a little optimistic) but certainly isn't 20 times the volume.
As Audioguru says though, you need a lot of extra power to get a small amount of extra sound.
You could try some extra capacitors on the output of your power supply but a 16v transformer at 2A isn't going to give you any more than 32 watts of total power output so you may well run into problems with three LM1875 ICs driven off the single transformer.