Is a jelco sa750e low mass or high mass ?

The Jelco SA 750 E has an effective mass of approx. 5 grams.
Adding a headshell you can adjust it from medium to heavy
according your needs.

I thought they were closer to 16 grams.  It does not surprise me that the Denon sounds good with that arm

Using Jelco 750E + Denon 103R w/ Soundsmith level 2 retip.  Sounds fantastic!

At 13.25 grams it would be considered medium mass. It should therefore work with a wide variety of cartridges. I’ve used it with a Denon 103 and it sang nicely.

Its a higher mass arm

I went ahead and dit it already.
Straight out of the box, it's really a revelation. Like a A-Ha moment.

I have a Jelco 750 with a Denon 103 and 103r both function just fine.

I have run a 103 on my 750L with good results, well as good as the 103 can do anyway.

I have used the 103 and its variants for years. Yes, it will work fine on a medium mass arm! The 103R is a better choice over the older 103! Excellent mc cartridges that punch well above their weight!