Whoops, forgot to mention that the XX2mkII is a LOMC with a gain of 0.28mv.
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While with some components doing what you propose might result in a number of issues, after looking at the specs for the PP4, the Ref Phono 2SE, the cartridge, and the McIntosh C2300 preamp listed in your system description (assuming you are still using it, as your line stage), and after doing some related calculations, I don’t see any problems in this case.
One point to keep in mind, though: I wouldn’t be surprised if the RCA output connector of the phono stage for each channel is wired directly to one of the two signal pins on the corresponding XLR connector, with the same output stage providing the signal to both of those destinations. Which means that having the PP4 connected while listening to your system **might** have **slight** adverse effects on the sonics of the system, since it would result in unequal loading of the two signals in the balanced signal pair. But given the 47K input impedance of the line-level inputs of the PP4, I suspect that won’t be a significant issue. Also, though, try to keep the length of the cables to the PP4 fairly short, and/or choose cables having low capacitance per unit length, as the capacitance of those cables (which is proportional to length, as well as being dependent on the capacitance per unit length of the particular cable) can also affect the signals received by the line stage preamp if (as I suspect) the RCA outputs of the phono stage are driven by the same output stages which drive one of the two signals on the XLR connectors.
P.S: An alternative approach, which should work well and would also eliminate the possible issue I described in the second paragraph of my post above, would be to connect the PP4’s inputs to the "SRVR" (Record) outputs of the C2300. Those outputs of the C2300 appear to be driven by dedicated output stages and are therefore independent of the C2300’s main signal path. And it appears that the signal levels received by the PP4 connected in that manner would be the same as the levels it would receive if connected to the RCA outputs of the phono stage.
Too much gain would result in either severe and obvious distortion, at least and especially on very high volume musical peaks, or (less seriously) in having to utilize your volume control undesirably close to the bottom of its range.
The KAB calculator, like many such calculators, is based on "one size fits all" assumptions that are either overly conservative with respect to many applications, or are completely inapplicable to some applications.
Note this statement in the introduction to the KAB calculator:
The optimum gain is based on achieving 325mV rms output at 5 cm/s. For the current crop of CD recorders, 300mV is required for 0dB recording level with the recorder’s level control set at max. Aiming for 325mV gives a little margin.Your NAD PP4 has a specified maximum input level for its line-level input of 5.3 volts, which is 24.2 db greater than 325 mv!
20 x logarithm(5.3/0.325) = 24.2 db
The specified maximum input level of the balanced line-level inputs of your C2300 preamp is 10 volts, which is 30 db greater than 325 mv! The C2300 has a gain from its line-level inputs of 0 db (i.e., no gain) to its tape outputs (which could be utilized to drive the PP4, as I indicated), and 15 db to its main outputs. The 15 db gain, coupled with the high gain of the phono stage, may (depending on the gain of your amplifier and the sensitivity of your speakers) cause you to have to utilize the volume control of the preamp in the lower part of its range. I haven’t taken the time to look into the numbers for the amp and speakers, but I suspect that you would still be using the control at reasonable positions. And I noted that you haven’t mentioned any issue related to volume control settings, even though you have apparently already listened to the balanced outputs of the phono stage with your system.
The REF Phono 2SE has huge overload margins. Its maximum output spec is 80 volts. 74 db corresponds to a voltage gain of 5012 times. 5012 multiplied by the 0.28 mv output rating of your cartridge is 1.4 volts. Although the output may exceed 1.4 volts on extremely high volume musical peaks (that would cause the cartridge’s output to be greater than 0.28 mv), perhaps reaching a few volts at times, it would still be vastly less than 80 volts.
The REF Phono 2SE has a maximum input rating of 250 mv. That can be presumed to apply just to the low gain mode. But even so, the difference in gains between the two modes of 23 db corresponds to a factor of about 14x, and 250 mv/14 = about 18 mv, which is still many times higher than the rated output of your cartridge.
As I said earlier, the assumption of the KAB calculator that the phono stage should provide an output of 325 mv when provided with the cartridge’s rated output is a "one size fits all" assumption, that is overly conservative with respect to your application. Personally I don’t use online calculators unless I can determine that their underlying assumptions are consistent with the specific application I am addressing.
P.S: I just noticed that the photo of the preamp in your system description thread shows it as being a McIntosh C220, rather than a C2300 as stated in the equipment list. However the specs on the two models that are relevant to my comments above are identical, so my comments apply to either model.