I think you should start with better headphones, like the Beyerdynamics Amiron Home which are probably a lot better than dt880 and warmer than dt1990. I have the Amiron and love them.
For listening through speakers it is really hard to say when a pre will help and not. You might have to lend one and try.
For listening through speakers it is really hard to say when a pre will help and not. You might have to lend one and try.