This is how I would come to a conclusion on this. Do I want the same sound just more loudness and authority? If so set a price range and research reviews. Look for something similar in sound and hope it interfaces like described in the reviews. But remember you need at least 200 watts to make a noticeable difference in loudness and authority From what I read of the reviews this amp (Primare I21)has a relaxed sound and some what diffused it lacks precision in both imaging and timing terms and it is not a tight sound. it could spread the sound wider than the speakers and do so in an effusive yet engaging fashion. There is a touch of exaggerated openness but this space allows notes to form a fuller image. Is this the kind of sound you have now? If not take into consideration what the Thiel 1.6's added or took away then try to mate a Amp. If it was me I would go with Parasound Halo A-21. See if someone has that set up and if it interfaced well.
Good Luck
Good Luck