I think you should buy first a pair of monitors (For example M-Audio BX5) and after if you need one - you should buy a subwoofer (M-Audio's subwoofer).
LX 4 is quite small system. The subwoofer has only 60 watts and I think that it's really underpowered. 8" woofer should have atleast 120W or more (my opinion... might be wrong). The only monitors I've heard from M-Audio is BX5 and I liked them. LX4 can't be much worse.
Truths are very powerfull, but the treble is quite odd. The titanium dome somehow "rings"... I don't know.. but they're 150% good for the price. And think they'd be better for mixing music.
But here's my tip: Don't trust any of the text you from the net. I've bought some products and been very disappointed, even though they said that they'd be good. Go to the shop - listen to some speakers and then decide.