I had the 5350SE and the 5i. Also have Vandersteen Models 1, 2 and 3's. The problem is the 5i. Its just not good enough for Vandersteen's. It doesn't produce clean power. Your Creek was a much better amp.
For now, if you back off on the tweeter 1db and run 2 sets of identical speaker cables and double biwire, that should make it more listenable. It doesn't have to be anything expensive, a small spool of Monster or AQ should be fine. Eventually, it would probably be best to switch to higher quality solid core cables, but not now because cables won't overcome the Naim's weaknesses.