The XA series amps tend to flush out more midrange magic, detail, layering and texture. To the point of being incredibly organic. The X series have lower and more authoritative bottom end, but you would only know it from doing an immediate back to back comparison. Same with the highs, they are more extended.
Now your question is a good one in that the X350.5 runs in pure class A to 40 watts, so it should be close. I have owned both amps at different times, so honestly I can't even say which one I prefer. Maybe if forced, I have to say the textures of the XA series are more preferable. If you have speakers that need 350 watts, get that. If not, get the XA60s. Not to mention the 350 is 150 pounds and a killer to move around. The XA60s are about 65 pounds each, very easy.
