If you just want to audition an MC cartridge, and if your Bryston already contains a hi-gain phono section, as your post suggests, then all you need is an MC cartridge. But it would be important to find out exactly how much gain is provided by the MC inputs on your Bryston, because there is a wide range of output voltages available among MC cartridges. As you may know, Low Output MCs (LOMCs) generally provide roughly 0.6 mV output (for a specific stylus velocity that is used as a standard in stating the voltage output of a cartridge) or less, sometimes much less. On the other hand, High Output MCs (HOMCs) generally make at least 1.0 mV, for the same standard stylus velocity. (There is no absolute cut-off between the two categories of LOMC and HOMC, but it's somewhere in that range between 0.6 and 1.0 mV, based on my own feel for how people talk about it.) For an LOMC cartridge, you want at least 60db of gain from your phono section, which will handle moderately LOMCs.
Yes, you also have to think about cartridge compliance and tonearm effective mass. LOMCs tend to be low compliance, as has already been said. For such cartridges, you would want your tonearm to be at least 11-12 grams in effective mass. (The combo of compliance and effective mass determines the resonant frequency of the system, which you want ideally to be between 8 and 12Hz. Formula is on-line.) However if your tonearm effective mass is too low, the good thing is you can add mass by simply using a heavy headshell or even just headshell meaty headshell screws. But don't think you can't enjoy combinations that appear to be a mismatch based on the math.
