One funny thing, even if a speaker is not rear ported, often times they perform best when given more space from the rear wall. Try giving them a little more room, like 24" to 36" and you may find that sound improves further.
That much room may not be practical either in terms of your room or WAF but it will clarify the issue completely. If that is the issue and you can't have them that far from the wall, then you can rule out Jade 3s
Also, the Jades are a very warm speaker in my experience. If you want more warmth, I am not sure that Harbeth or Spendor will be warmer. You may find that an amp change is going to be more practical and/or effective.
That much room may not be practical either in terms of your room or WAF but it will clarify the issue completely. If that is the issue and you can't have them that far from the wall, then you can rule out Jade 3s
Also, the Jades are a very warm speaker in my experience. If you want more warmth, I am not sure that Harbeth or Spendor will be warmer. You may find that an amp change is going to be more practical and/or effective.