I honestly can't think why there would be a difference, it's a bit of wire that does the work.
I meant to say slow blow/time-delay as well. 3.15amp. If you do the maths (and I can't remember the equasions) but taking its rms value of 505watts, a 2amp fuse covers 501watts I think it was. The peak power it can draw however is 1000watts, so I'm thinking surely that 2amp isn't enough.
You could use a 1000amp fuse and the speakers would be fine. The only reason you have a fuse is for when there's a surge in the mains supply, and surges just don't happen these days, or happen very very rarely.
3.15A seems to be spot on, it's completely safe. Also, the decoder is known to be quite fragile, so you don't want a fuse blowing on it and give it a jolt all the time.