At 4.5mV output on your MP-150, you’ll likely be running way to close to the overload margins of a phono stage running at 59 dB - especially a budget phono like the Mani. So in your case, yes adding a headphone amp with a net gain of around say 10-20 dB (careful to find an optimal volume position for whatever you choose) and running the phono at a lower gain would be much more optimal. But it’s still less optimal than just running a single preamp with a good gain structure for your system.
Doesn't your Mani have a 48 dB setting? that should be optimal for your MP-150. If your system is lacking gain at that setting, then ideally you need either more gain in your preamp, more gain in the amp, or more sensitive speakers.
Doesn't your Mani have a 48 dB setting? that should be optimal for your MP-150. If your system is lacking gain at that setting, then ideally you need either more gain in your preamp, more gain in the amp, or more sensitive speakers.