Oh, that hurts... yes, I did a major study (at least 8-10 different blocks), and after a couple of years I still owe the results, at least to the people at Alembic who were kind enough (is that the right word?) to get me started.
Unfortunately, one thing I had no way of testing was the difference between an original, non-routed bass, versus same with the addition of a block.
However, there are some older posts here from people who added a block to their bass, and some searching should turn them up. As a start, I suggest searching for all the words sustain block add, which should get you a few.
From everything I've read here in the last few years (which is very close to everything), I do not recall anyone regretting the installation of a sustain block.
Of all the things I tried, brass was really very good, hard to beat. Again, I can't compare with/without, but I generally think it will improve the sustain, pretty evenly across the tonal range. At least compared to other block materials, brass was one of the most neutral in increasing sustain, without changing the overall balance.
There is some slight chance that brass in particular will bring out the highs just a tiny bit more than the mids, but I doubt you could tell without doing a controlled, side by side comparison.
I don't know enough about how you play or what you like, to advise you. But just going by the words, fretless and beautiful midrange rich sound, I think you'll like it. It will help all those overtones play against each other a bit longer, making it more rich and complex without noticeably changing the balance.
Try a search or two anyway, to see what others have said. I'm not sure you'll find anything quite as specific as your question, but you might pick up some useful observations.
