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I just discovered that there were other options for figure-eight mics for M/S and have found very little opinions on the blue line stuff from AKG.
For about $1300 you can have a cardiod condenser and a figure-eight pair for essentially half the cost (new anyways) of a mkh30/40 pair. Plus there would be new capsules to buy to add a shotgun or hyper.
I take it they are in the awkward mid-tier price range where people are usually gonna go for gold or go home kinda thing.
I've heard some concerns about noise but that's really about all I can find.
On the contrary, I own two blue line bodies and cardioid and hypercardioid capsules for each. They're great mics, but, yes, the stereotypical Sennheiser and Schoeps mics are definitely nicer. They do have a higher noise floor, such that I wouldn't want to use them as an M/S pair. I don't mind using them in coincident or spaced pair configurations for stereo recordings, but I'm not convinced I'd be happy with the resultant noise floor from an M/S summing procedure. That's just me.
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Could be just marketing (mouth to mouth and retailers) as well. Some brands and product lines are just so widely used that there's no excuse to look for alternatives.
I've used the Blue Line series as a cardioid x-y pair with the knuckle/swivel mounts for location work as they fit well into a single suspension. You can't compare them to a schoeps/senheisser solution but for the price I was mostly happy with the results.