First off, it's your mic, so it's your call, obviously.
Also, if you're able to undo it, you'll likely be able to redo it.
I would strongly advise against it, though. After the treatment, your U67 will no longer be a U67. It will basically be like the countless modern mics with seriously hyped tops. Well, with better parts than most. But nothing special really.
If that's what you're after, you could also get or build such an animal from scratch.
The U67 circuit is a very sophisticated design. Lots of brainpower went in it. And there's nothing like it.
And of course the K67 capsule was designed with that intricate high frequency filtering in mind. The highs in the K67 are pre-emphasized, so without de-emphasizing the capslue will be overly bright.
A healthy U67 should not sound dark BTW. The original plots are flat, with a very slight rise even, depending on polar pattern. They have been called dark, but that would be as compared to modern LDCs, many of which are not flat. Also, quite a few, if not most of the originals are tired to some extent and should be sent to an expert. They will sound fresher upon return!
Of course you could install a stepped filter. or you could use an equalizer to tame the highs after the filtering circuit has been removed. And yes, often filtering is better than amplifying.
But there are exceptions to that rule and the U67 is one of them. The U67 takes lifting of the highs like no other microphone. That is, if you have a good equalizer.
And with the circuit intact it acts like a(-n active) de-esser (as if there's a compressor in the side chain). Or should I say a de-eshjzer? There's undistorted sibilance and there's distorted sibilance. I'm talking about the latter. Again, here's a paradox. The U67 has lots of negative feedback and often that would mean a less clean/fast/pure path. But it was put there deliberately and the whole of the circuit (with nothing left out) fights distorted sibilance and similar artifacts that can occur in condenser mics. And consequently you can lift the highs more than you would expect, without getting spikes and filth.
This is my miserable way of explaining, mind. Others, like top recording engineers who have worked with U67 when both the mics and the men were in their prime, have done a better job explaining it. Still, it remains an intricate design, made by true masters of the craft (who were given almost unlimited financial means for designing), so it's not so easy to grasp.
Apart from the filtering system as discussed above, there's also a broadcast filter. You have the DU67 and Dany's version is true to the original, so the broadcast filter is there, too. Another U67 cloner here decided to leave it out from the start, which makes his not true to the original in that regard. But that's a lot less radical.
So IF you decide to go ahead and take filters out, I would start with the broadcast filter.
But again, it's your call.
Henk
