I'm not in the know, but trying the Gold Lion12au7 might help. They are not so expensive that they are a major investment.
Another option is talking to one of the highly regarded tube sellers, the names of whom you can find in the thread, "Where do you guys buy tubes?", just below this one. One of these guys may be familiar with your amps and the effect a particular tube might have on the sound.
It may be that your DeHavilland amps just have a softer sound. Either way, good luck and let us know what you end up with.