Welcome back! Your instincts are good---go for a used Mirafone! They are reasonably priced, hold up great, keep their value like no Chinese ISO (That would be an Instrument Shaped Object, in the words of Lee Stofer!), and play as well across the line as any tuba on the market. I recommend you contact some of the other tuba sellers here on the board, preferably by phone--reason being that there are horns not listed on their web page that are available oftimes. People like Bloke, Lee Stofer, Matt Walters at Dillon Music, David Fedderly at Baltimore Brass, Tony Clements, Roger Lewis,Dan Schultz,etc. can pick out and ship a wonderful horn and you will have confidence that you are getting a really good horn. Or, invest a few goonies and fly to one of these stores and pick one out for yourself. I also recommend NOT going the e-bay route if you don't absolutely have to; you may get lucky, then again you may not, and the cost is just too high. There are horns listed in the "for sale section here, too, that you may want to look into.
Whatever you do, enjoy it!
