I have been reading information on AVSForum for about a year to help with understanding how to get the most out of my older system and help me on my upgrade path but I only joined up today because of your question. Please do yourself a huge favor and go find the YouTube channel called:
Home Theater Gurus
The guy that runs it is a member here on AVSForum (sorry I don't remember his username off the top of my head), he presents his topics step by step and provides the basics for beginners but also provides all the detail required to understand the topic at a more advanced level. For your Sub question and how to get started with multi-sub and miniDSP watch episode 8 then 7 (at the start of episode 7 he asks you to stop and watch 8 before 7). Then watch the whole series from the beginning, it contains a massive amount of good info on every topic related to home theater.
Enjoy your new gear!
5.2 Speakers - Front Mirage OMD28, Center Mirage OMD-C2, Surround Mirage OMD15, Sub - PSA TV18, AVR - Yahmaha RX-V2600, Amp - Parasound 2250 v.2, 4K-UHD - Panasonic DP-UB820, Display - LG C9 65", Other - miniDSP 2x4HD, Monster Power HTS2600, PS4, 9865HDC (cable box)