A DSD remaster such as 'Let it Bleed', originally released on SACD, is available as redbook only CD and sounds much better than earlier issues. It's no match for the actual sacd in air, etc but very good in bass weight and texture.
The krell 400xi is extremely neutral and extended. If you want to fill out the bottom a bit on older recordings, I recommend leaving the krell and speakers alone (keep that bandwidth!), and consider upgrading to a better (possibly tubed) digital source, especially one with user selectable treble filters. Choosing one with a steep slope roll-off may enable you to goose the volume on the 400xi when listening to the leaner recordings in order to fill out the bottom without becoming to bright in the process.
The krell 400xi is extremely neutral and extended. If you want to fill out the bottom a bit on older recordings, I recommend leaving the krell and speakers alone (keep that bandwidth!), and consider upgrading to a better (possibly tubed) digital source, especially one with user selectable treble filters. Choosing one with a steep slope roll-off may enable you to goose the volume on the 400xi when listening to the leaner recordings in order to fill out the bottom without becoming to bright in the process.