I recently acquired JCM900 4501 combo.
I love it but it's versatility can be questioned.
What i am wondering is how much will an eq in my effects loop give me.
I'm mainly looking for more bottom end.
any suggestions?
The EQ in the loop will make a major difference.

I boost the level of the EQ, then boost the two low-end sliders one notch, while cutting the highest frequency almost all the way down.

I'm using a BOSS GE-7. Works pretty well. There are other higher-end EQs, like MXR. You'll find that an EQ enables you to dial back the gain on your amp as well. Less gain increases note articulation.

Have fun.
'68 RI LP Custom
'90 JCM900 4100 + EQ & Boost
A lot of Marshall combos are funky like that. Don't be afraid to crank the bass to ten, Mid at 6 and Treble at 5. Try that before buying an eq. If that doesn't cut the mustard get good quality eq. Don't get the cheapest. An eq will definitely open up new doors on the amp.