I agree that it sounds like a good idea to change both dac and pre-amp. If you want one integrated unit or two separates are up to you. I did take a look in the latest TAS, issue 321, since it has a buyer's guide. There are a lot fewer on-box solutions than dacs or pre-amps, you will have less to choose from but maybe get a more cost effective solution.
They did list NAD C658 and Nuprime DAC-10H as combos. The latter may be more of a dac with volume, I don't know if it has any analog inputs or if you require any.