Okay so i need a bit of advice. My bass player has purchased a kustom de 300 hd head and cab to suit.However when he turns the amp on for a gig the small fuse at the rear keeps blowing. We've put a spike protector on the plug and an rcb, but to no avail.Any advice,mad guitar chap. Thanks.
Is this happening at the same venue each time? Because if it is, then it is quite possibly the venue's wiring that is the culprit. Beyond that, since the DE300 is a hybrid with at least a preamp tube, it could be that the tube is bad. Check it out.

Here is what the good folks at Mesa have to say about this issue:

"Maybe this world is another planet's hell?" - Aldous Huxley
If you blow one fuse then it could be just the fuse, though that is unlikely. if you blow two then there is something wrong with the amp. If you put more fuses in without putting the fault right then you might break the amp even further. The fuse is blowing because the electricity is going where it shouldn't, it's a crude but effective way of stopping too much power going into your amp.

It could be the tube, and it's a good idea to carry a spare anyway so try replacing it by all means. If it fixes the fault then throw the old tube away. If not then you probably have a faulty component inside the amp or something has shorted out part of the circuitry. Unless you really know enough to read a circuit diagram and use a meter I doubt you'll fix it by yourself and the voltages inside the amp can hurt you and even kill if you are really unlucky so take it to a tech unless you are 100% confident in your knowledge.