I own and play all of the above, and you've received some sound (pun intended) advice all around. Much has been written about the Conns and Bueschers, so I'd like to add a few things about the other two FANTASTIC American makers, Martin and King.
I have my pick of horns, especially in the Tenor department, but have, without hesitation, made the Martin Committee Tenor my main ax. They do indeed have a remarkable core sound-very lush, deep and beautiful. They are a bit more focused than the Conns, without losing the dark tone, and the intonation is a bit better. Altissimo is a walk in the park, and the key work is fast, light and ultra smooth. These horns are beautifully built and engraved, but some of the most solid horns you are ever going to see. They are less mouthpiece fussy than comparable vintage horns too.
The Alto through Baritone models are all winners, and they can still be had for less than some of the other pro horns (although this is rapidly changing as we speak..those prices are starting to soar).
Many King models are really underrated horns presently. I have horns from the 20's H.N. White "King" up to the Super 20 Silver Sonic models, and all have that remarkable, scintillating presence of tone that Kings are noted for. The mid 20's Kings are nice horns, with good intonation (as Pete so astutely mentioned, a lot of the supposed intonation "issues" some report is due in large part to using an unsuitable MP setup), great tone, and key work that although requiring some finesse to set up properly, is surprisingly modern and efficient compared to other makes/models of this period. I'd seek out a horn with more modern key work given a choice if we are talking about making it your primary horn however. Just make sure you get a model that post dates the 1924 improvements that eliminated some of the octave vent and intonation issues with the earlier models.
The King Zephyrs, post early Voll True II based models and pre-Eastlake vintage, are fantastic in all regards, and great value for money. Every bit as good as the Super 20s, sans a few cosmetic frills. Both the Zephyrs and Super 20s are monster players, and will blow the roof off of just about anything out there. Regarding volume/ projection.... the sky is the limit.
As always, choosing from among so many great horns is a very personal trial, and the only way to know in your heart and mind which model is a perfect match for YOU is to play them all, preferably side by side.