I've heard the M&K S150 speakers, but not the Triads.
I didn't think the M&K were any great hell to be honest. They came from the days when Dolby Digital was the best sound quality we had, and THX was screwing up theater audio with their misguided theories on vertical dispersion and completely ignoring comb filtering and lobing. They're like, "here's a speaker, and here's a speaker, and here's a speaker! Oh, you want a seamless soundstage, a realistic recreation of a live concert space, natural, transparent sound that mimics real life sound? Well too bad. We're busy drawing your attention to each individual speaker one at a time here. At least it's painfully clear though, right? Except when higher fidelity recordings than lossy Dolby Digital come along and lay bare the limitations of our inexpensive, massively marked up drivers. Oh well, Skywalker Sound used to use us! Not any more. But we'll just keep marketing ourselves that way, regardless. We're MK Sound, dammit! Why innovate or strive to get better when we can just sell our name to a Danish conglomerate, keep the same 35 year old designs, use cheap parts, improve nothing, and sell at an insane markup based on name value and the heritage of speakers that used to be cutting edge 30 years ago, but now pale in comparison to modern designs?"
Or should I tell you how I really feel?
Yeah, no, honestly, the S150 are fine as mixing monitors when you want to focus on steering your mix into each channel one at a time. That's what they were made for. That's what Skywalker sound used them for. But as a playback system in a theater, they're not my cup of tea AT ALL.
I've no idea about the Triad Gold speakers.
You've got a small room though. And it sounds like it is nicely acoustically treated. I'd be hopping all over some super high resolution, wide dispersion speakers with lightning fast transient response. My pick would be the Ascend Acoustics Sierra Towers and Sierra Tower Center (aka. Horizon Center), all with the RAAL ribbon tweeter upgrades. Some Salk or Philharmonic speakers using a very similar RAAL ribbon tweeter would be good alternatives as well. There's a bit of personal taste, but those three are all contemporaries. I favor the Ascends, other folks might favor the Salk or Philharmonics. I like the smaller footprint of the Ascends for a small theater like yours though, and the Horizon Center is superb! I also LOVE the bamboo cabinets used by Ascend go for a finish that shows off that lovely bamboo if you can. With any of them though, you'll be amazed by what you MK S150s have been hiding from you. Not in raw detail, but in soundstage, imaging, and realistic recreation of real life sound quality! Even with your strictly movies listening habits