The Marantz has a poor phono preamp compared to the unit fitted to the turntable, even Michael Fremer rates it quite good but he even says it could be improved on.
The Technics SL1500c is NOT semi automatic where did you get that idea from, the only thing it can do and you can switch it off is raise the tone arm at the end of the record.
First of all Michael Fremer is an audiophool wrong more often than by chance. You weigh the cod's wallop that he espouses by the ton and not the ounce.
Now, as I said, an RIAA phono circuit is a very simple circuit and very unlikely to have significant errors.
If you hear a difference and assuming it is a difference the problem is very unlikely to be an error in the RIAA Eq circuit.
LP reproduction is tricky, and there are opportunities for things to go wrong.
One of the big issues is cartridge loading capacitance.
All cartridges have an optimum loading capacitance. This used to be in the specification and now seldom is.
To make matters worse, there is no agreed phono loading capacitance. The biggest variable in all this is the capacitance of the cables from cartridge to phono input.
So when you use the built in phono amp in the turntable, the connection will be low capacitance because the length of the cable will be short.
When you connect the cartridge direct to the phono input you have a longer cable and the loading capacitance will be higher.
So SME were aware of this and they used to use very low capacitance leads. Back in the day, when people had skills, you looked up the specified loading capacitance of the cartridge. Then you added the requisite cap at the base of the SME arm to make the required optimal loading capacitance. That assumed the owners had soldering skills. I have been soldering for seventy years now.
Peter Walker of Quad, put dip switches on top of the Quad 44 preamp, so that sensitivity and loading capacitance could be optimized for the owners choice of cartridge and turntable.
So I suspect that for that cartridge, as you have found the built in phono amp is the more optimal of the two, because of the short connection between cartridge and preamp.