£400 to spend on a first DIY system

[VvvvvV]

Hello,
my room is strewn with carcasses of old fairly cheap speakers, and I have been mixing and matching anything I can get hold of to make a decent loudspeaker.
The truth is though, when I whack up the volume the sounds becomes emaciated and the gangly, one of the speakers starts to fart the bass and the old floorstander just about rocks the bed around, and mid range vanishes.

I think it is time to start investing in the best sound available, from scratch. What is the best way I can spend 400 pounds on an amplifier and a clear image with about 150 watts of subwoofer
and 150 W of midrange etc for a small room?

I'm interested in any solution that can sound as good as some really high quality floorstanders at high volumes, although I would be really keen on having the same quality as a couple of studio monitors with a bass woofer.

I like to scavenge for kit, where should I scavenge around the UK what exactly can I buy, and should I really donate some of my internal organs for an amplifier ?