Conn Constellation 24-I Euphonium on eBay
That is an excellent Buy-it-now price for a 24I that appears to be in top condition. I purchased mine a year and a half ago for around $900. A refinished straight bell (25I) horn done in polished silver plate (never an original finish for a Conn of that vintage) sold for $2,000 recently on eBay.
Your comments about the Connstellations are spot on. I still enjoy playing mine and can get a really nice sound on it. I find it does not respond that well to the Wick 4AY, but plays better with a Wick 4BS (shallower cup). The intonation with a deep cup Wick like the 4AY starts to get poor and the Connstellation actually plays better with a bit more back-pressure.
While different sounding from the modern compensating horns, they make a very sweet sound and in the right hands do not sound at all "trombone-ish".
Sterling Virtuoso 1052HS & Besson Sovereign 967 euphoniums
Sterling Virtuoso 1050HS baritone & Conn 24I/25I euphonium
New England Brass Band/New England Wind Symphony
New England Tuba Quartet/Nashoba Valley Concert Band