Phono amp compatibility with integrated

Happy Holidays Everyone!

I appreciate all the feedback that you've given me recently on phono stages and tonearm/cart compatibility, so I'm back to ask for even more advice.  I've had some issues with two different phono stages that have "high" and "low" output settings (the Jolida JD9 mk2 and the Heed Quasar).  When running the Jolida into my old Simaudio i-1, I found that the high setting easily overloaded the amp and caused distortion while the low setting seemed dull and lifeless.  I've since moved to a Naim 5i-2 (LOVE it!), and I have had the same issue with both the Jolida and the Heed Quasar.  I have since returned the Quasar.  I have a Vincent PHO8 as my back-up, and it has only an MC/MM button and no adjustability.  It works in all circumstances that I've encountered - no overload with any preamp or integrated amp, no distortion at high volumes. 

In a previous post, Almarg mentioned that my integrated has a "high sensitivity (= a low sensitivity number, 150 mv in this case) of your Naim integrated amp."  I don't really understand what this means in terms of finding a phono amp that is compatible. What specs should I be looking at to determine the compatibility with the Naim?  Output impedance?  Gain?  I was hoping that you might be able to give me some guidance or point me to a site that explains how to figure out this type of matching.  

Thanks so much, I wish all of you a great holiday season,
Scott