Buying in Canada, bringing back to US

I have found scads of threads on this question but I can find no easy answer. I've looked at government website, other forums, and it's astonishingly confusing. 

I live in northern New England and Montreal is the closest city. Here's the basic question: if I drive into Canada, buy a NEW piece of stereo equipment, and drive back into the US, what will I pay in taxes, tariffs, customs, duty, whatever the fees are? (Will I pay taxes in Canada, too?) And what if I buy a USED piece of equipment? (Let's assume at least $4000 CAD.) Does it matter if the item was manufactured in the US? 

I've spent a fair amount of time looking at the US government's website. It's ... overwhelming. It looks like the "rate of duty" for loudspeakers is either free or 35% (quite a difference). There simply has to be a simple answer. 

I've crossed the border many times and I know they don't always check, etc etc. But I would declare whatever I buy. And also: I'm not talking about having something shipped. 

Thanks! I know this has been discussed out here for over a decade. But I am hoping someone can tell me in simple terms what the duty (tax, customs, tariffs) rates are for stereo equipment in 2020.