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Hey there, first post.

This is my first attempt at composing for a large ensemble. I started it today and it's obviously a work in progress. I'm also not yet sure what instrumentation I want to use, so the strings might be temporary. Any suggestions at all are welcome!

I've personally never been a fan of calling something a 6th chord... it's just an inversion, but what you have is fine.
1. Held melodic lines are always risky business, since the surrounding envelope of the note becomes harmony to the rest of the orchestration. Just be careful with what's underneath the whole notes.
2. Whole notes in general are good, and they create a platform to rise up from. However, a repeated figure really shouldn't use them because it actually depresses that same platform. Perhaps instead of a repeat, change the inner voices the second time around!

Good start!

Interesting enough, but only If you find room for some diversity in the continuing development. Are you scoring this for string orchestra? Such a boring sound in my opinion. Large ensembles are interesting for their lush sound and diverse timbre. Since strings are all basically different sized versions of the same instrument (with exception for the Bass) the sound is very homogenous and the larger the ensemble the more homogenous it sounds. This, of course, is the point. And it works well in full orchestras, but in string orchestras the tendency for everything to blend together and sound flat is a very real danger. Nothing is really popping out, so it all turns into static. For purely string ensembles, I find that chamber groups are more effective. Individual instruments can speak out more clearly. Of course, that is not to say that composing for string orchestra is pointless. It has pitfalls however that need to be overcame. A lack of timbral contrast leads to a stronger need for textural and dynamic contrast.