Changing arm length for DUB crank My bike has a 175mm crank arm (https://www.sram.com/en/truvativ/models/fc-styl-6k-b1) on a Specialized Epic HT (https://www.vitalmtb.com/product/guide/Bikes,3/Specialized/Epic-Hardtail-Expert,27186)
I think the important words are (don't know their meaning): boost, threaded, dub,
Now I want to try shorter arms 170 and even 165. Since I don't know exact length I'd like, I'd like to try cheaper ones if I can, try them first and buy a more expensive one once I decide the arm length.
I read DUB cranks are compatible with any existing BB (https://www.mbr.co.uk/news/product_news/sram-dub-370517). What I want to know is if dub BB is compatible with non-dub cranks?  Otherwise my options are pretty limited to SRAM cranks it seems.
No, DUB cranks are not compatible with any existing bottom bracket.
DUB is the deign of the crank spindle and the bearings in the BB. A dub crank only fits in a dub bottom bracket. It’s possible to get DUB bottom brackets that fit in many different frame bottom bracket shell standards, that may be where the confusion originates.
If you want different cranks you either have to get DUB compatible ones, or replace the bottom bracket along with the crank. There are many different crank and bottom brackets that fit in a threaded bottom bracket frame shell. If you choose to go with non-DUB cranks, you will have to deal with other compatibility issues such as getting the proper crank chainline. You’ll also need some special tools or a repair shop’s help to un and re-install the bottom brackets.
In your position, I would look for a used SRAM DUB crank that is a drop-in replacement for your current one; possibly one in rough condition or missing the chainring that is relatively inexpensive. If you do not like the shorter arms you will be able to sell it for approximately what you purchased it for.