Roland Fantom ?

With latest version the system can be set to basic or advance; ADVANCE allowing you to play 16 internal zones (having 4 partials on each left/right channel for a total of 8 waves per zone) and 16 external devices simultaneously.

Go to the exact opposite end of the digital spectrum, so you've got something that's truly complimentary to the Montage, with very little overlap, and opens up whole new worlds of ease of sound design and spectrums of sound design: the ASM Hydrasynth:

https://www.youtube.com/watch?v=Tm35lajXOdE

Plus it's got an incredible keyboard. Maybe the best synth keyboard with after touch, ever.

And a good sized ribbon, a much more traditional arpeggiator and most insane modulation matrix outside of a PC.

Depends on what you're after. Korg + Fantom are romplers with some modeling engines. Either is slightly different than Montage. A more analog-centric keyboard may fit the bill. Not sure what you need or want for extra sounds. There are so many choices between modern and "antique" gear.

For a MIDI controller, the Native Instruments MK2 has a nice keybed and supports 16 zones.

On one hand I'm thinking a very basic midi keyboard would fit the bill

Keep in mind that, if you want to "take advantage of the other 8 parts" of your Montage patches, you'll be limited by the number of MIDI zones that very basic MIDI keyboard supports. Typically, the basic MIDI controllers only support 1 or 2 channels of transmission at once, which would limit you to playing 1 or 2 of those 8 parts. For a full 8 parts, you'll have to look a little beyond "basic." Jason's suggestion of Native Instruments could work (I believe the model is Komplete Kontrol S, not MK2). At that price range, you could also look at boards with sounds... the Korg Kross would do it, and probably so would the Roland Juno DS. Though the NI probably has the best action of the bunch.

on the other hand I was thinking maybe a Kronos or a Fantom or some such. If I add another serious synth I would want the one that gives me the most of "what the Montage doesn't have".

Kronos and Fantom are both good choices, with plenty of MIDI flexibility. Both add VA (virtual analog) synth, which the Montage doesn't have. Some of the main things that Kronos has that Montage and Fantom do not are a modeled clonewheel organ engine, full on-board linear sequencer with audio tracks, and the ability to use very large sample libraries that stream from disk. I think Fantom has nicer interface/ergonomics, including dedicated VA synth controls. I'd probably also give the Roland the edge over the Korg for many acoustic instrument sounds (now that an update added a bunch of "SuperNATURAL" sounds). It also has specific integration with Logic/Mainstage, if that might matter.

I left off the S designator as this is the # of keys. I did intend to list "Komplete Kontrol" but didn't - oops.

KOMPLETE KONTROL S49 Mk2
KOMPLETE KONTROL S61 Mk2
KOMPLETE KONTROL S88 Mk2

NI (Native Instruments) refers to the whole series generically as "KOMPLETE KONTROL Mk2 keyboards"

It's really difficult to really suggest any keyboard that makes sounds since I have no concept of what you would possibly want.

Ah! My googling of native instruments mk2 brought me to their maschine mk2 product!

Yeah, I know - it's been confusing to me too. The "MK" designation is also used by Novation. They have the ##SL MK2 and MK3 where ## is the number of keys. There are other Novations with MK2&3 too - but the Novation SL line is the NI Komplete Kontrol S## Mk2 competitor.

If Novation supported more zones I would probably choose it as a pure MIDI controller as it has more buttons/knobs/sliders on its control surface. Even with a much worse keybed than the NI. Both of them have the LEDs above every key that can change color to indicate zones. I prefer this so I can keep track of where my sounds are if I forget. Haven't used the feature yet - but it's good to have. The Novation SL has a lot of stuff under the hood. Really wish they'd bump up the zone support. I'd trade dealing with a lousier keybed if it had this.

As 61 keys able to drive multichannels :

- Roland Fantom
- Korg Kronos or Nautilus

But, If you want to enhanced your sonic landscape :

- Novation Summit
- Sequential Prophet 6
- Arturia Polybrute
They would be a real complement to your Montage.

The Novation SL has a lot of stuff under the hood. Really wish they'd bump up the zone support. I'd trade dealing with a lousier keybed if it had this.

I have no experience with it, but their site says it supports 8 zones. It's not 16, but it's enough to address parts 9-16 on a Montage, so could also be fine for the task at hand.

I had a choice and decided to pick more zones + better keybed (IMO) over the control/sequencer flexibility in the Novation. Pretty close to flopping over to the other side until the final purchase decision.

This is ultimately going to be a personal decision. So your (plural) choice may not have the same criteria.

First thing to look at is the keys..

If you have76 or 61 keys, go for something with hammer action..
Having a Montage 8, i am quite happy with having it paired with a Genos and Mainstage or Camelot Pro.

But you specifically asked for a Synth? Then i would say, you defiantely needs something with a VA or true analogue engine.
The wuestion is which synth would offer you most?

- Nord Stage 3 compact, 73 keys, basic VA Synth, lovely ORgans with drawbars, and some nice extra Piano sounds.
- ASM Hydrasynth, great Wavetable synth, with poly aftertouch, did i say poly aftertouch, like in polyphonic?
- Roland Fantom which has VA and some lovely Roland synth models but then why would you want 2 fullfledged wotrkstations, which have much in common?
- Novation summit, just because its one of my favorite synths.
- Roland System 8, a very tactile Roland synth with great use.
- Poly Brute by Arturia, a great verstile synth, with a ribbon and a touche controller
- Any Dave Smith synth, just because they are all great.
- Yamaha Reface CSX just because they are quite small
- Korg Wavestate, because it offers a synthengine not available in VST.

There is no best choice, there is just preference and reasoning.