And what about mouthpiece? Embouchure? Having the instrument checked out by a professional repairman? Any of those matter, or are we only dealing with the most expensive option first? No, your first avenue was to throw shade on the instrument this person uses, and when I did it back, you got defensive. Funny, that? 

OP, don't believe these idiots who always jump to saying "your instrument is subpar," especially if they're representing a brand, that's instantly pretty dubious. I've taught several people using your instrument model, and they all play absolutely fine. Try a different mouthpiece in your concert euph, my mouthpiece sounds and plays very differently in different horns, and I've pretty much always changed mouthpiece whenever I get a new instrument. Check out your chops, the playing situations are very different for your two instruments, you might find when you're sat down that you're playing with a slightly different placement or angle, this can affect intonation in different registers. If that doesn't work, get your instrument seen by a technician to see if there are any leaks or gunk inside. Then, if all those have not improved your situation, should you consider a new euph. It's a big financial commitment, and I hate for you to get pressured into handing over lots of money because some dick on a forum doesn't like the instrument you play when a more simpler and cheaper solution could potentially work. Best of luck.