I'm not a Fishman fan, but I'm curious as to what problems you had with the Baggs iBeam. I like that pickup quite a bit in the iMix system to compliment the Element, but not quite so much on it's own. I've also found it's tone to be a bit unpredictable on anything more than the most standard bracing and plate system, though it can sound great on some guitars. It's also very sensitive to placement - i.e., it can give a terribly unbalanced response if an edge is touching a brace.
The iMix is still my current favorite in the bang:buck ratio. The Element coaxial saddle transducer is much less tinny and quacky than the Fishman piezos, and more dynamic than the ribbon transducers in my experience. Their M1 active is pretty impressive too if the customer doesn't mind a soundhole pickup.
I've not heard the D-TAR Wavelength, but it appears to be a similar transducer system to the Element (which I like), but with more headroom and flexibility to tailor the sound than the Element active system. Hopefully Rick will chime in (and don't worry about sounding like you're jumping in to plug your products here - I asked the question and am curious
).There are a lot of K&K fans around, but I've never liked them myself. They've always just sounded like disc top transducers have always sounded to me - not the best clarity or distinction, way too much mids and too low feedback resistance. Then again there are plenty who swear by them, so maybe I'm just not using the right preamps. Preamps - of course remember that's a necessary other half of the system.
You're obviously going to get a lot of opinions from a lot of different tastes. It might help to know what style of guitar this is, what the player's styles and goals are, what environments it will be used in, etc. There's no single perfect system, and this stuff is all important to guide your pickup choice.
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