Hi,
I'm an Australian high school student who's coming out of Year 12 and I want to buy a new horn.
My first horn was a Elkhorn by Getzen and my current horn is a Yamaha 664.
I like the 664 a lot, and it seems to favour a light, warm sound (e.g. Mozart) but I feel, especially during band practice, that it becomes quite lacking when attempting to play loud, dark, and warm. When I try loud, dark, and warm, the tone becomes very shallow and brassy. I've thought of 8D, because apparently they have a good, warm, dark, epic horn sound, but I hear they have a very heavy tone and I still want to be able to play light and warm.

Currently I'm thinking of:
Conn 6D - the baby brother of 8D, I've read, and thus a bit lighter.
Holton H178 - Similiar, a nice average horn
Yamaha 668 - I've heard this is also similar to the 8D, but not sure on how closely.

If it seems like I'm going for a Jack-of-all-Trades, yes, but it's more like master of none. I want a nice general horn that doesn't really specialize and will play decently, but not excellent, for almost everything.

My price range is intermediate. I want a second hand horn, and probably don't want to go past $2000. I live in Australia, so shipping is going to be a fortune either way, so don't worry about where it is. I'm also willing to chip in a bit extra if it means extra quality.

If anyone can give me any recommendations, that would be great.

I'd keep your eyes out for a bargain. You'll see in a post I'm about to write, that I just acquired a Paxman custom (2 made by Bob Paxman, designed by Merewether in the 60's, one for Roland Berger, the other for Tony Chidell) Double Descant from the '60's for close to what you've got. Don't discount brands like Alexander, Knopf, and Kruspe, as out of your range. Often their compensating horns, which unlike student compensators are completely professional, are light, very responsive, and play like a dream. Most importantly, much cheaper than a full. Houghton has a very nice compensating Kruspe for less than 2000, so keep your eyes out.

Why not trying to restorate your horn?
i'm playing an alexander 102st feom the seventy's its a good horn and i can say for sure that an yamaha 664 matches up to it. if you want a darker sound you could fir example make the bell detachable and buy a heavier model. that alone should already do the deal. i'm visiting the musikmesse in frankfurt every year and i'm playing myself trough the whole stack of horns every time so i can assure you that you have a great instrument already in hand. sometimes little changes are more useful thand replacing your whole instrument