i don't really think that's a great idea to be honest- you're buying something which is mediocre and which won't really do what you want, then throwing another several hundred pounds at it to try to persuade it to do what you want- why not just take all that money and buy something of higher quality which is aimed at what you want (or at least is aimed much closer)? add up the cost of the VK head (£275), VK cab (£200), four celestion speakers (~£200), new valves (£100?) and a tubescreamer (~£100) and you could just get a better amp. I mean, an engl screamer is £700-£800. already has a good speaker, probably doesn't need a tubescreamer as it has plenty of gain already. and just is a much higher quality amp than the valveking. you could also look at a laney tt50 combo, which again you could easily get for that amount of money. or save very slightly more (another £200-£300), and you're into the really nice stuff indeed.
i mean, if you want a sports car and have enough money for a porsche you don't buy, say, a mondeo and then throw £20k at it to try to make it do what a porsche will do (and will likely do better, even with that £20k of upgrades) straight out of the dealership... i don't know too much about cars, so that analogy may be way off, but you know what i mean. :)
