I currently use a yamaha 4 valve and i find tuning hard, but possible. I use lots of different valve combinations and this really helps. e.g. 4th valve on top G. Other than that, the sound is really nice if you get the right player on it! and i find it depends on the mouthpiece. i have a huge mouthpiece, which gives a really nice sound, but no high notes. So i'm currently compromising on a bach 7C, which is fine.
And, having played a euphonium most of my life, i just cannot get my fingers around using 1 and 3 for bottom d. My fingers just won't do it!