Yes this is normal. When 6db's are gained, your volume level will have to be decreased from what it was as you have gained more in detail and lowered your noise floor which translates into blacker backgrounds as you have described.
As for the anti-cables IC's, keep in mind that they take a long time to break-in(200-500 hrs.with 500 being the optimum) so be patient! A lot of folks aren't and that's why you see some negative reviews on them because they're not given enough time to break-in. They will sound good right out of the box but they go thru a metamorphosis over time(between 100-150 hrs.) where they will sound soft in the bass and restricted in the highs but after 400-500 hrs. they will sound better than any cable out there costing 35- 40times more.
As for the anti-cables IC's, keep in mind that they take a long time to break-in(200-500 hrs.with 500 being the optimum) so be patient! A lot of folks aren't and that's why you see some negative reviews on them because they're not given enough time to break-in. They will sound good right out of the box but they go thru a metamorphosis over time(between 100-150 hrs.) where they will sound soft in the bass and restricted in the highs but after 400-500 hrs. they will sound better than any cable out there costing 35- 40times more.