Question about d440 connection between master-slave

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1. if I am using a coax digital cable between master and slave, that is all I need in theory?

Yes, that should be all you need. Oh, as well as another SD card for the companion, of course.

2. If I want to add the tether cable, can someone give me recommendations on brand/make?

I think you're referring to the trigger cable...?  In which case, I'd say just find something cheap and cheerful; mine is by the brand "Syncwire" and seems reasonably well made.

3. If I want to use an RCA-to-XLR cable instead of digital RCA-RCA, do I need to use the tether to activate the XLR input on the slave?

No, you can use RCA-to-XLR without the trigger cable -- at least, I have done so in the past.  However, the Configurator is a bit fiddly in this area and you have to do things in the right order to end up with a working configuration.  I'm sorry to say I can't give you the exact sequence off the top of my head, but I think it was detailed in another thread (possibly by @Daniel.avasilichioaei?).

Good luck, and I hope you enjoy the 440!