I have used Koetsu Rosewood Sig, Benz Ruby II, Shelter 901, Lyra Helikon, Benz Glider(0.9mV), ATOC9, etc over the years with ARC PH3SE. One of the major strength of the ARC PH3SE over other high end phono preamp is ability to change the sound via chaning 6922/6DJ8 tubes easily and there are plenty of different type of 6922/6DJ8s to try out. If you don't feel you have enough front end gain for ~0.4mV to 0.5mV cartridge, contact ARC and get resistor sets for your LS15 to increase the gain on your preamp(12dB single ended gain to 18dB single ended gain).
Make sure to experiment the resistor loading on PH3SE via listening. Cartridge loading for your set up will be different than other people's set up due to your music preference, rese of set up, etc. Also try out different set of input tubes on both PH3SE and LS15, you will prefer one set over the others on certain cartridge.
From what you describe, Rega P25 with RB600 arm, turntable/arm combination may become the weaklink when you upgrade to higher performing cartridge.
Make sure to experiment the resistor loading on PH3SE via listening. Cartridge loading for your set up will be different than other people's set up due to your music preference, rese of set up, etc. Also try out different set of input tubes on both PH3SE and LS15, you will prefer one set over the others on certain cartridge.
From what you describe, Rega P25 with RB600 arm, turntable/arm combination may become the weaklink when you upgrade to higher performing cartridge.