hi guys thanks for your suggestions.
At first I thought it was being caused by an new outboard compressor I've just bought, but when I tried the mic directly plugged into my soundcard, the hiss was still there. I use the same set up all the time and don't get such noticeable hiss, so I don't think it is the laptop.
The SM58 even without Alias Factor has hiss, so I think its just the SM58.
I might try the RODE NT1A and then put Alias factor on that to try and beef up the sound.
Could I maybe emulate the SM58 beefiness someway with some cunning eq and tube?