I agree with Matt that you are unlikely to have a problem, but the question cannot be answered with certainty without knowing the make and model of the line stage, the power amp, and the speakers. There are two possible issues that need to be considered, one being the possibility of overloading the line stage, and the other being the possibility of having to use the volume control undesirably close to the bottom of its range. Chances are neither will be a problem, as I say, but without knowing the characteristics of the line stage, the gain of the power amp, and the sensitivity of the speakers it’s not possible to say for sure. (BTW, in general the possibility of overloading the phono stage itself should also be considered in such a situation, but assuming the relevant specs of the IOS are similar to those of the Ref version, which I was able to find, that won’t be an issue).
FWIW, 64 db of gain corresponds to multiplication of the cartridge’s output by 1585 times. So 0.6 mv would be boosted to 0.95 volts. Particularly high volume dynamic peaks on some recordings may increase those voltages several-fold, which would result in the output of the phono stage reaching several volts at times. If you have one of the relatively few line stages which has significant gain in the part of its signal path that is "ahead" of its volume control, and/or if the line stage or the power amp has relatively high gain, and/or if the speakers are particularly sensitive, there could be a problem. While there won’t be a problem in most cases, those possibilities are probably why you have seen conflicting opinions on similar questions.
Regards,
-- Al
