A lot can be done with speaker placement and listening chair placement to improve sound such that room treatment need not be extensive and ugly. What you should do may depend on any restrictions you have on such placement. For example, if you are forced into a placement near the side walls, treatment of those walls may take more priority. If you are going to be seated relatively close to the speakers, much of what you hear will be coming directly from your speakers and reflections off the walls, ceiling and floor become less of an issue. Consider speaker choice in conjunction with you placement requirements. For example, if the speakers must go in the corners, look for ones that work well there (e.g., Audio Note).
You should take advantage of the help the treatment product firms offer in planning your treatment. Whatever they say, start slow and do your treatment in relatively small increments so you don’t overdue things and end up with a dead room. I would start with non-treatments that are practical and decorative. Bookshelves that you might need to hold books, records, whatever, do a good job of breaking up reflections off the wall. Decorative tapestries do the same. Artificial plants (this is a basement), such as ficus trees do a good job as room treatments.
There are way too many possibilities for speaker choices so I will limit my suggestion to mentioning Volti speakers; if you like the Heresy, you will probably like the Volti offerings and they are a substantial upgrade to that sound.