Hi Jarkko,
Here are some links to Isabellina customer feedback on the audiocircle forum - which might be helpful in describing some of its sonic attributes:
http://www.audiocircle.com/index.php?topic=58678.0
http://www.audiocircle.com/index.php?topic=69303.0
http://www.audiocircle.com/index.php?topic=63514.0
There are comparisons to other dacs in these posts, but I don't remember if the Weiss was compared.
The Isabellina dac webpage also links to a review from Positive Feedback Online.
We offer a 30-day in-home trial period, so maybe you can try both in your own system and see what sounds best for you (do a shoot-out and see what comes out on top in YOUR system).
Please feel free to contact me if you have any questions.
Best regards,
Vinnie Rossi
Red Wine Audio