0.48 mv compared with 0.3 mv is a difference of about 4.1 db, calculated as 20 times the logarithm (base 10) of the two voltages.
Which means that for equal volume levels you will turn up the volume control on your preamp to a point that provides 4.1db more gain with the 0.3mv cartridge than the setting you would use with the 0.48 mv cartridge. Which in turn will, in the case of the lower output cartridge, boost the noise/hiss levels that are generated at the front end of the system (primarily by the phono stage) by 4.1 db, relative to what they would be with the higher output cartridge.
That is a significant difference but not a huge one. Although if your phono stage (whether separate or in your preamp) has noise/hiss levels that are marginal for use with low output cartridges, it might mean the difference between acceptable and unacceptable hiss levels.
However, I see in your system description that you are presently using a 20XL, which also has a 0.3mv output. So going to a 0.48mv cartridge would result in your setting the volume control 4db lower, rather than higher. That would REDUCE front-end generated noise or hiss, which is usually the predominant source of noise or hiss in a system because any noise generated there is amplified by everything that follows.
The concerns resulting from using extremely efficient speakers would be:
1)That you don't want the overall system gain, and the output voltage from the cartridge, to be such that you end up using the volume control near the bottom of its range. If you are not presently approaching that situation with your 0.3mv cartridge, you should still be ok with a 0.48mv cartridge.
2)That noise or hiss generated by what follows the volume control (i.e., the power amplifier) is acceptably low when reproduced by the speakers. Which is a separate issue that is unrelated to the cartridge and the phono stage.
Hope that helps,
-- Al
