Interesting. Any idea how my cartridge would perform, do you believe it would cause an issue? I have the rega exact, here is a link to the specs. Not sure if it gives you enough to go on though
http://www.rega.co.uk/exact.html
When the manufacturer only lists 6 technical specifications, including among them 'colour' and 'fixing', it is difficult to generalize/hazard a guess, other than to say that with an output rating of 6.8 - 7.2mV it should play loud enough without having to twist the knob fully clockwise!
Leaving subjective comments aside, objectively, it is important to make a distinction between cartridge output/phono input sensitivity and gain vs. cartridge electrical loading requirements for best performance.
WRT output/gain; with Naim's comment that
'the gain is perfect for 5mV moving magnet cartridges', combined with their history of providing 40dB of overload margin, it is hard to imagine that a worst case scenario of an Exact, even one with a high side 7.5mV output, would cause overload distortion; certainly, it won't be as hair trigger as CD on some Naim amps either.
WRT phono stage input loading, it is more a case of optimizing frequency response at HF vs. avoiding audible ringing in extreme cases.
Not a very 'exact' method, but reading between the Rega product specifications should suggest a preferred range. Fono MM input loading, for example, is listed as 47kΩ in parallel with 100pF capacitance. By comparison, Brio (2017) is rated 47kΩ in parallel with 220pF. In either case, on the loading side one must add in tonearm cable capacitance. The old RB300 'improved' grey lead measured 85pF from RCAs to arm pillar plug terminals; plus roughly 15pF internal, so circa 100pF. Therefore, an Exact into Fono MM should be presented with a capacitive load of 200pF, whereas, into a current Brio the total is 320pF. I suspect that Rega have chosen these values plus a high overload margin (40dB), as they want their phono stages and integrated amps to appeal to a wide market regardless of whether or not their own very high output MM cartridges are used (a lowish 2.5mV, as specified for Carbon, for example).
Whether an Exact would perform as well with the 470pF of Naim (circa 570pF total) is a question worth pursuing before committing to purchase. A fellow who posted on the Graham Slee forum measured the internal resistance of both his Elys and Exact and then estimated the internal inductance by plotting the impedance/frequency graph. He measured 165R and subsequently determined 100mH. With the latter value it is possible to calculate the resonant peak frequency for various capacitive loadings. For example, combining an internal inductance of 100mH with 570pF puts the HF resonance peak up to 21.1kHz, well above the audible range. For reference, the resonant frequency is given by the following equation (where L = internal inductance and C = capacitance):
Typically, the larger the internal inductance, or the larger the capacitance, the lower the frequency of the resonant peak within the audible band (also increasing in amplitude along with the reduction of frequency point); however, dramatically lowering one variable whilst increasing the other can have the opposite effect, as is the case of Exact into Naim above. That is, assuming an internal inductance of 100mH to be correct.
P.S. This is the closest thing to full specifications that Rega (via their American distributor, The Sound Organisation USA) has published for dealers...
https://soundorg.com/content/Rega_Info/Rega-Full-Line-Comparison-Chart-July-2017.pdf