After a bit of assistance here with some troubleshooting, my Crack is working great. It seems to need a bit of a warm-up, as I get some crackle noises for up to 10 minutes or so at the beginning, so I turn it on early and leave it for a while. Other than that, all seems good.
I've got the speedball waiting to install, but I will leave that for a few months first so as to get a feel of the amp without it.
I'm wondering though, if I'm going to put the speedball in, is there any other upgrades that could go in first so I don't have to take out the speedball just to do them later.
This is more a matter of convenience that trying to push the amp as far as it can go, as I expect the speedball to provide a massive improvement anyway.