not surprised at the op’s findings -- rotel is mid tier... will not bring the best sonics (that is why their higher line called michi exists)
a truly great preamp will result in great sound, arguably better than running a system without one
but even a decent preamp, and certainly a mediocre one, will detract from the purity of sound compared to none at all...
whether any particular volume controlled dac will sound better into any specific power amp relative to having a line stage in between cannot possibly be generalized... the specifics will drive the outcome, specifically
-- the nature of volume control in the dac, and where in the operating range the dac volume control operates in normal use
-- the output stage of the dac, its ability to drive the amp
-- the input stage of the receiving amplififer and how well it mates with the dac (or pre) feeding it