I am considering to purchase a Conn 8D. Would it be better to get it with a spit valve or not? Would it have any noticable effect on the sound? Also, would a screw bell have any noticable effect on the sound?

Ah, the ever famous Horn questions. No and Yes; or Yes and No.

Spit valves - I have avoid them for years. But quite honestly I have had to get them as more and more things I do leave little room for emptying the horn. Does the spit valve affect the sound? Technically I think it can but honestly when done correctly not enough for anyone to notice. I suggest an Amado water key. They are designed to fit flush with the curved tube hence less effect on the air stream. There is a correct and incorrect way to do it so I hope your technician knows the correct way.

Solder it on and then drill the hole. And make sure it is at the correct angle that you will be using it.

Detachable bells - Again lots of arguments on this one. Some people can't tell the difference; I believe I can. Anytime it comes down to picking between a screw bell and a fixed bell in blind tests I have always chosen the fixed bell. How many people in the audience are going to hear the difference? I don't know. The detachable bell is great for traveling, but it does affect the sound. That said, there are lots and lots of professionals playing on screw bells.

All my horns are fixed bell except one, and that was done specifically for airplane travel. Am I happy with the sound? No. But, it is serving it's purpose.

Since I don't think I will go far enough with horn to run into somebody who would mind the sound of a detachable bell, I think I'll get one. Is there anything I should know about taking care of a horn with a detachable bell?

Pencil graphite makes a great and inexpensive lubricant for the screwbell threads. You might want to make a mark on the two rings showing where the threads line up (just a small dot or something. When you put the bell on gently thread it and then add just a little more to homd it in place but not so much to make it stick there. You don't want it to unthread while you are holding it, but you want to get it apart when you are done playing.

You might have to wait a few minutes after playing before unscrewing, some people have found playing heats it up enough to make the threads bind, letting it cool down releases the pressure.

So just scribble over the threads with a pencil? That sounds too easy, but it makes sense.

Yes, scribble with a pencil is the most inexpensive way. PErsonally I don't like it. It doesn't work as well for me as others I know who do use it.

The important thing is to use a dry lubricant. There is dry graphite available. Check your local hardware type stores and see what they have. regardless of the price a dry lubricant will last a long time. (or use the pencil trick).