I don’t know for sure but maybe someone with an 800S can comment. My guess would be that you could add a little bit but you may run in to the excursion limit of those dynamic drivers. They will definitely not have the slam of the Focals or Audeze but it will be better.

It is also worth noting that buying used is a great option. Typically speaking, a headphone loses a big chunk of initial value once it is purchased but after that the price stays pretty flat. So assuming you are patient and pay reasonable prices for the headphones you can easily use them for a few months and then sell them and buy another headphone in that range until you find what you like.