My first guess as to the source of your problem would be improper loading. Although the manufacturer of the cartridge may specify a particular loading, in reality, the proper loading differs from one phono stage to the next so one must experiment with different loading. When a cartridge is not loaded down enough (remember, the HIGHER the number, the lower the amount of loading--e.g., 47k is a very high number, so the cartridge is not being loaded down much), it will sound bright, edgy, sibilant and thin in the bass. Let's say you loaded the cartridge at 200 ohms with one phono stage, in a different stage, that same number may be way off the mark. So, experiment with loading before dumping the cartridge.
Choice of tubes can make a BIG difference so some experimentation there could yield a satisfactory result. If you are substituting the original tubes with 5751s to lower gain (your phono stage uses 12AX7s?) you could also get other sonic benefits because those vintage tubes sound pretty good. This substitution usually can be made without any problems, but, you might want to check with the manufacturer because it is not just gain that is different; the operating parameters are slightly different.
You also mentioned concern with lower output cartridges and RFI. If you have RFI concerns, you must be certain that all your interconnects carrying small signals are shielded cables. Also, I have found that cartridges that loading down a cartridge (lower number value for the loading resistor, e.g., 100 ohms or lower) dampens the top end response of the cartridge and significantly improves RFI rejection.
Choice of tubes can make a BIG difference so some experimentation there could yield a satisfactory result. If you are substituting the original tubes with 5751s to lower gain (your phono stage uses 12AX7s?) you could also get other sonic benefits because those vintage tubes sound pretty good. This substitution usually can be made without any problems, but, you might want to check with the manufacturer because it is not just gain that is different; the operating parameters are slightly different.
You also mentioned concern with lower output cartridges and RFI. If you have RFI concerns, you must be certain that all your interconnects carrying small signals are shielded cables. Also, I have found that cartridges that loading down a cartridge (lower number value for the loading resistor, e.g., 100 ohms or lower) dampens the top end response of the cartridge and significantly improves RFI rejection.