I'd like to add a dissenting voice on the Rath R9. The open tube was okay, if a little more woofy than I was looking for when I tried it, but adding either trigger created instant bark at higher dynamics, and a corresponding obvious change of tone quality at lower volumes. I played around with it for half an hour or so, but couldn't find a way of coming at it that avoided this problem. Consistency of tone quality across the range is a must, and this instrument just didn't have it. Perhaps I picked a bad specimen. I would imagine that the Tenors are better, as the trigger register is not so important...
I've tried various Bachs (indeed, I played on one for 8 years), and have gone off these - they just punish you too much for any slightly imperfect blowing. When I moved onto the Holton TR181 that I currently play, I couldn't believe how easy it was to sound good on it. I've tried various Holtons, and been impressed with each of them. Oddly though, I hear that Holton Tenors aren't so good - opposite to the Raths?
What else? Edwards - I've tried a few of these (though I haven't tried their customisation procedure), and again, been far more impressed with their Tenors (gorgeous!) than their Basses (same problems for me as with the Rath). Not too keen on their Altos either...
Shires - close to the Edwards (but then, isn't Shires an offshoot of Edwards?), but more consistent, for my money, and also even more expensive. An absolutely massive sound (a bit OTT, maybe), and rather hard work.
Yamaha - breaks up too easily, but not a bad instrument for brass band stuff. The one I tried really didn't get on with my Bach mouthpieces, but responded much better to one it came with.
Conn - both modern types 60H and 112H (I think those are the correct designations - one is single trigger, the other double) seemed very nice, and quite similar to the Holtons, though a little brighter, which was quite refreshing in this age of immense Bass Trom sounds.
Sovereigns I haven't tried for years, although a chap I know owns one of c.1980 vintage, which is quite nice, although the trigger setup is a little odd.
On the subject of valve types, I have found very little consistent difference in response and tone between different models; perhaps it is to do with the way I blow - I try to play with as wide a lip aperture as possible.
Why not buck the trend - buy a G Trombone!
Dave