I think it is time to upgrade my high school student to a double that could carry him through college. Looking for advice on brands/models, used/new and best sources for best deals.

What is your maximum budget for a horn? I'd suggest up to 4000$ for a proper new double.

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Here's a good buying guide I found: http://www.corno.it/Articoli/SoYouWantToBuyAHorn.html

My suggestions sort of depend on what your high schooler might be thinking of doing with music. If they are just going to play for fun or be a music education major then I would suggest the Holton 179 or 180, Conn 10 or 11. Yamaha pro-series.

If your high schooler is planing on going on to play professionally then I would suggest staying away from the Holton line altogether. If capable the Conn 8D is a great horn but difficult to find a good one these days and difficult to truly play well. Lots of professionals are playing the Yamaha 668 series (try to get a 668N). If price is not a problem Schmidt is really popular right now and he is making good horns, but they are expensive. Durk is finally coming around with some good designs and excellent reviews.

I would encourage you to check out:
Pope Instrument Repair. He has good fair prices. He is very knowledgable man.
http://www.poperepair.com/

Patterson Horn Works.
http://www.hornworks.com/

Dennis Houghton has a great selection of Conn 8D's
http://www.homelesshorns.com/Sales.html

And don't forget to check out Mark Atkinson. This man has horn making in his veins.
I'm having him do some special lead pipe and bell work for me.
http://www.atkinsonhorns.com/

I think John meant Schmid, not Schmidt.