Several good suggestions and some not so good.
The LTA may be a great preamp for some but is clearly not what the OP is seeking.
I would agree that if the OP is at all troubled by too much edge or brightness this is probably a function of his speakers and will probably not be mitigated by a new preamp. You need to fully love your speakers from the get go. Component matching beyond should be subtle tuning of the system to taste.
Lastly, I have heard several speakers with more than enough bass that had an intolerable edge. I do not agree that brightness is a function of poor bass response.