The arm itself has little effect on volume. That is almost entirely down to cartridge output. The arm does however have wiring, and connections, and those could produce a small effect. A bigger effect though might be vibration control. Remember the signal you get is the sum total of all the vibrations- stylus, cantilever, cartridge, headhshell, arm tube, bearings, mount, arm board, base, plinth, platter, motor- all of it. Its not just the record. Its all added together. The better arms of course add a whole lot less.
Now virtually all these vibrations that are added by the cheaper arm, they are there but at such a low level as to be inaudible. In other words you would never listen and go wow that vibration, it really bugs me, I need a better arm.
This is a huge mistake, yet people do it all the time. I don't need a conditioner my power is clean. I don't have a problem with this mess of tangled wires they sound just fine. On and on. Just because you don't hear a noise and go "Oh! There!" doesn't mean you don't have noise. It simply means the noise isn't distinct enough to be heard as noise.
So what happens instead, and what you could be hearing, is the Jelco is so much better it has dropped the noise floor to where you feel you need to turn the volume up a bit. Not saying that's what it is, just saying that's what it could be. Because it happens. Happens all the time.
Especially depending on how they achieve the better performance. There's a balance between stiffness, which preserves dynamics, and damping, which eliminates noise. Done together the music has even more presence and life and dynamics because the lower noise level has revealed more detail. But it can also be done with more damping, in which case it might still sound better but with not quite as much dynamism or life.
What makes it hard to know is going from one arm to another changes a whole bunch of things at once. Not as much as changing a turntable and arm and cartridge, but still a lot. That's why you can learn so much with tweaks like fo.Q tape. With that you are changing just one tiny little thing at a time. You can even control where and how much. You can if you want use just one inch and move it around listening and learning which areas have the greatest effect- those are the areas vibrating the most.
What you will learn by the way if you try this is the whole damn thing is vibrating like you can't believe!
