Regarding that the 660s is more intimate with kinda worse imaging and soundstage imo. Separation also tends to be better on the 1990 for the most part
The 660s isn’t as technically proficient but it is a more relaxed and warmer signature, pretty good for just relaxing
Regarding the 1990, what pads are you using? Also I would wait and use it for a few more days before you make a decision to get familiar with it and also let it break in and get used to the signature
An alternative is that you could get a tube amp for the 1990 if you wanted which would make it less harsh and relaxed imo