First question for me, is you have both a Subtank mini (which is a RBA), and a RBA atomizer.. do you mean you have a subbox and a subtank?
Second, usually, if you vape until you get a burned taste, then you have burned out the cotton in your RBA. Once your cotton is burned, you cannot "unburn". The burn taste will continue no matter how much cleaning you do. The cotton is burned, and you have to replace the cotton. If you're a DIYer, you can
take the coil out and replace the cotton. or, you can
replace the whole coil(and cotton) as the easy fix like most people do.
Like others suggested and you've already tried, I would clean the coil out and give it a shot first. On a few rare occasions, the cotton isn't burned that bad. but if you've vaped it dry, and continued, I would bet that the cotton is too far gone.
In general, vaping till it's dry is a bad thing for just such a reason. When it gets low, refill or put it down until you can refill it. Many times when it's real low, it doesn't wick properly, so replace it before it gets down to the white cotton area and you should have a coil that lasts a good while. Let us know how things go.