Should I get the Moon 891 streamer/dac/preamp for my Moon 861 amp? Or separates?

More information would be really useful. It would really help us to see your system, if you create a virtual system and photos.

Given what you have asked. The real question is where do you want to go sound wise?  

Right now your MacIntosh is probably supplying some warmth that an all Moon system will not have. 

Really the most important thing is to identify the "sound" you are going for. On one end of the spectrum is highly detailed, lean, somewhat analytical systems with good slam. On this side is also what I call "sonic spectacular... crazy holographic imaging". On the other end is highly musical, natural sounding with great imaging, midrange bloom, great rhythm and pace. Combining different house sounds can make a perplexing number of different sound types. Moon is pretty detailed and a bit on the analytical side. MacIntosh is a bit on the romantic musical side... but lacking in detail. Your current combination may be pretty good. Focal tend to be hot on the top... so you Mac will help calm that down. 

I am not a fan of combined streamer / DAC / Preamp systems. They are too much compromise... depending on your expectation. Note for instance, what is probably the best stream on the market is the Aurender N50 a three box solution, a great preamp is the two box Audio Research Ref 10. You get the idea... great sounding components are single function.

I would recommend a Aurrender streamer... N20 or better. And a good DAC... in the $ 7 - 10K range. I would love to recommend the Audio Research Ref 9... but it is getting long in the tooth. Berkeley Alpha is a great DAC. The DAC can be a critical decision because, like a preamp, it can influence the tonal balance... detail / musicality. It is easy 

Before I could recommend a preamp. I would need to know more about your tastes. It is the key... and I think you need to be very careful. 

@dman777 what do you mean by best sound from your amp? what is your setup lacking audibly to you? What do you want to add or subtract. Bass, midrange, treble, shimmer, sibilance. And note  a preamplifier is the last one to change to fi any of the above. if you haven’t gotten the speaker, room, listening position dialed in. You can change Preamps as you want. You still won’t "get the best" out of your speakers

Seems you've already made up you mind though so get the Moon Preamp

Well, I’m not getting tight base and the clarity and detail that I heard the 861 in the store. Of course there’s other variables. But the only thing I can think of is the streamer since it’s the cheapest. What I hear is a lot of bloated base and many tracks will sound flat

well the obvious underlying factor here is the different spaces. Your room isn’t the store. Your room dimensions differ from the store. Your furnishing is different. The distance you listen at  in your room is surely different from where you sat in the store demo

As you can hear deficiencies in bass and some bloat. it is a room related issue. Basically room modes differ in different rooms. Someone’s null might be your bloat. and you can’t tackle this by changing preamp. That is an impossibility. You need to tackle room/speaker coupling, seating distance (so you’re not sitting in nasty bass nulls)

If you continue to fly blind. Yes you may buy the Moon Audio but a different Preamp doesn’t solve room nulls (that is an impossibility. Physics can’t be whished away with purchases)

So, you’re better off  knowing the approximate tonal identity your room in imparting under 500Hz in your room first. What are your room dimensions, listening distance and distances of speaker from front wall and side walls? I can simulate something in REW @dman777