I have no experience with the Line 6 Helix, but it certainly looks like a beast - well built and all that. I didn't watch out for guitar effects in the past couple of years, or effect boards that have been manufactured with guitars in mind in the first place, except maybe for the Pigtronix Infinity Looper, which is still on my list.
At first I was wondering where the stereo inputs are, but then I learned that returns 1/2 and 3/4 can act as stereo input blocks, which is good. What I would personally like about a box like this is that you can switch everything with your feet. It's not precisely what you'd call affordable although it offers a lot. The thing is, though, that most of the stuff is aimed at guitarists and I'm not sure how much you would profit from that unit as a synth player. You might not need it at all for your Prophet-6 and OB-6 at least. Given that also the Pro 2 and the Prophet 12 provide distortion and delays, I would maybe look out for a more specialized unit for reverbs especially, just like the H9 for example, which also offers nice delays.
Together with a synth, the sound might also get a bit too mushy in conjunction with the Line 6 Helix. I mean, think of how relatively simple the clean output signal of a guitar is compared to a synth patch.
In the end it of course depends on how much of an effects nerd you are, meaning whether you like effects of all kinds (Flanger, Chorus, Distortion, Overdrive, Phaser, Pitch Shifter, Ring Mod, Fuzz, Tremolo, Uni-Vibe, Leslie, etc.), or if you're mainly looking for selected types of effects.
I personally am very plug-in based when it comes to global effects like Delays and Reverbs, since my stuff is always set up in conjunction with Ableton Live anyway. So there I use Valhalla DSP stuff (Room, VintageVerb, and Shimmer), and u-he's Satin as a tape echo emulation that also does nice choruses and flangers. My only current hardware effect box is Moog's MF-104 Analog Delay. The only stuff I'd be interested to add (apart from the above mentioned looper) would be either a H9 too, or a Strymon BigSky, and this little fellow:
http://www.knasmusic.com/products/moisturizer/moisturizer.php