Hello Imederos,
I have a Sx900 that I play with a Fantom 8 (weighted piano action), and other times with a Fantom 07 (synth action). I also had an MODx in the past.
It’s a personal choice which synth you choose since both MODx and Fantom are great keyboards, and each has advantages and are very deep. I personally found the Fantom a bit easier for me to get my head around. One thing I really like about the Fantom is the “keyboard switch group” function of the 16 pads. You can assign different voices to each of the 16 pads making it very easy in a live setting to easily switch to 16 different sounds (with sound remain) as you play. On the lowest row of pads I have one pad assigned to sx900 voice 1, another to right 2, and another to right 3. So at the touch of a button I can play any of the Sx900 voices on the Fantom keyboard. So, as an example, if you are playing a flute on the Sx900 voice 1 and see that voice 3 is a sax, at the push of a button you can play the sax on the Fantom (while still keeping the flute active on the sx900 keyboard). The option of having 2 key beds for live playing makes it more enjoyable for me to decide on the fly what voices on each that I want to play. And it’s quite easy to do once you get the hang of it.
I have the Fantom on the bottom since it gives me 76 or 88 keys, and has dozens of very good sounding pianos, eps, etc. The SX provides all the arranger and pad backing, with the exceptional Yamaha acoustic voices. The Fantom has some great supernatural acoustic voices and many synth voices- over 6000 highly customizable voices in total with plenty of faders and switches. Roland’s midi integration is excellent, and the organ emulation is excellent (perhaps not as nice as my Hammond, but pretty darn good if you like organ). So with a midi cable, your sx900, and a Fantom 0, you have a powerful 2 keyboard setup that is very light and will keep you busy for years making new setups and sounds. Another thing I like is when playing at home, you can plug the Fantom into the sx900 speakers and it sounds pretty good!
I hope that helps!
Best,
Mike