Definitely warm your ingredients. Also, make sure that your chocolate IS in temper. I would have it at the high end of the temp. scale - but definitely in temper. If you're doing a large slab of bark at a time, your streaks are most likely due to improper cooling rather than the chocolate not being in temper to start with. I have success with having a fan blowing on the cooling slab for a good 20 minutes. Also, you want to make sure that you're not making your barks too thick.
If I were you, I would temper your chocolate in your tempering machine to the high end of the temp. range then scale out the amount of chocolate you need into a pan, add your warmed ingredients, stir, slab and turn on the fan!
I don't use a tempering machine. I use Mol d'Art melters. I don't imagine doing it in the tempering machine would work out so well! But I don't know enough about them to say for sure. I'm just thinking that there would be inclusion 'jams' going on.
I sometimes use the oven to melt out chocolate. My oven goes as low as 100F so it's great for melting chocolate overnight. If you don't have that option on your oven, just use your oven light and put your chocolate pan quite close to the light. Your chocolate should be melted if you leave it in overnight. Then just temper by hand using seed. You'll be pouring it out as soon as it's tempered so no issues of keeping it in temper.
Do you temper the chocolate before you make the ganache? I've learned that ganache needs to be made with tempered chocolate to get the proper consistency.
Have you tried making your bark thin and sprinkling the inclusions onto it? Like mendiants, very large mendiants.