You have to give us some kind of application... are you in a home studio, on the stage in a large arena, how much power do you want?

Skip the bass amp... completely
First, it is mono, your synth is stereo
Second, it is designed for an instrument whose lowest fundamental note is 41Hz and whose highest fundamental note is around 400Hz and whose highest harmonic is around 4kHz.

The synthesizer goes lower and way, way higher... a piano’s lowest fundamental note, for reference, is 27.5Hz and the highest notes 4,186Hz (higher than a basses highest harmonic)... acoustic piano harmonics top out at the top of the human hearing range 20kHz.

In short, a bass amp is not designed for full range (it specializes in bass, after all).
You want a FULL RANGE sound system.

Studio Monitors are best for listening at home. (Near field monitors).
Powered Sound Reinforcement cabinets are best for the stage. (Loud speakers).

While stereo may not always be practical, it does always sound better than mono. It depends on your application.

I don't want too much sound. Just enough to fill my small bedroom. I'd have to stick to the bass amp for now until I can save up enough for actual speakers. Now that I've explained my situation, what brands/types speakers would you recommend?

I use a bass amp for practice sometimes. It's frequency response causes problems making the keys shine - but it does in a pinch and practice is not a time where my sound is paramount beyond other considerations. Like what I can fit in my vehicle - what I'm willing to carry - and if the venue we practice in already has some form of sound reinforcement already.

So I get it.

But for the home studio where I'm doing sound design and want to enjoy the sounds of my keyboard - the bass amp doesn't come close to cutting it. Here I use HS series studio monitors. This is one of many suitable choices for the studio environment. And I'd consider your bedroom as your music studio.

Yes its is my little bedroom studio but I want to use these speakers strictly for the MOXF. My budget is $200. Are there any good studio monitors for 200?
If I buy one do I need a subwoofer too?

I'd say that on a budget, you never need a subwoofer for a bedroom studio.

For that budget, there are options - but I'm not sure what's suitable in that range. $400 would be HS5s. The used market shows HS5s at $200/pair if you're willing to invest the time into fishing for that price. I see online retail shops value the used at $300 - but those prices fluctuate and there's only one pair for sale at the moment (where I usually check).

I see other monitors in the $200 range for other monitors. KRKs tend not to be transparent - but some like the frequency boost profile of these monitors. I can't vouch for how they complement MOXF.

Thanks for your help! I'm currently checking out the PreSonus Eris 4.5. Both speakers together are just about $200.

I just put a pair of KRK Classic 5s on my MOXF8 in my small music room (dining room). I think they sound quite good.

The JBL 305P MkII are currently on sale at many places for $200/pr. I think that would be my choice for the price.