If you swap in the 4490, do you notice a big difference in sound quality (presuming that both have been powered on for a while, and just moving the input & analog output connections)
Hard to answer.
The Bifrost 2 is in my basement rig, and hasn't been powered off in a few weeks.
Bifrost 4490 is at work now.
When I was doing my initial A/B/C of Bifrost 4490, Bifrost 2, modi multibit, I was comparing toslink. None had been kept on for longer then 6 hours.
At that point I got a bit tipsy and forgot what input on saga belonged to what DAC, and ended up with a semi blind test.
Bifrost 4490 was the easiest to pick out because it didn't have the fullness of sound that the multibit dacs have.
At the time I really really didn't know what DAC was what, even though I thought I did.. but I'll say this.
The 4490 has awesome detail retrieval, toslink only, use it at work every day. No USB card. It doesn't quite have the fullness of Bifrost 2, and the Bifrost 2 in toslink has a bit better decay and attack, notes hang longer.
I've since figured out a better source for USB then what I was using. Bifrost 2 with unison is indeed magical (scotch mod). Been a listening to my test playlists on Qobuz tonight. I just switched over to Steely Dan's Gaucho, on 24/96. Wow. Soooo good.