I ran into a similar issue when trying to find a phono preamp to go with my Naim gear. Although the input overload margin was high for my Naim preamp I was finding anything greater than 60dB gain seemed too "hot" and the sound suffered. I could never get a good reason for this either from Naim or Klyne. Both pointed fingers. But ultimately I figured that since Naim amp/preamps have considerable gain overall that finding something similar in gain to what Naim phono boards were generating was the way to go.
Then there's the opposite when I was using a MM cartridge with a AI M3A and a CJ MV60SE. Even though the MM had a really high output, 6.5 mV, and the M3A's MM boards had 28dB and a total of 58dB of gain the cartridge seemed overly anemic in bass. When I added a sep. phono preamp and went to 50dB of gain did it seem to sound correct. Ans then there' my friend who had a M3A with differing amps and a ClearAudio MM cart. (3 mV) and had no issues. Go figure.
Unfortunately, you really don't know about interactions until you try them. I too wanted to try an EAR 834P. I've only heard one once (Vol. pot ver.) with an MC directly to an amp. The vol. pot was really touchy and probably defective but the sound was awesome, at least for classical music.
If you are not willing to take a chance then go for a unit that has adjustable gain settings.