This is a review of a Klipsch RP-600M which has been upgraded to a new crossover, binding posts and "No Rez" by GR Research. The kit costs US $244.
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OK, so there is nothing externally which is different.

I can't open the unit to show the different bits but here is the back with new binding posts (two are in parallel):
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I performed all of my testing using the standard binding post.
My old
review of RP-600M did not have the latest measurements so what you see is me adding those measurements.
Measurements that you are about to see were performed using the Klippel Near-field Scanner (NFS). This is a robotic measurement system that analyzes the speaker all around and is able (using advanced mathematics and dual scan) to subtract room reflections (so where I measure it doesn't matter). It also measures the speaker at close distance ("near-field") which sharply reduces the impact of room noise. Both of these factors enable testing in ordinary rooms yet results that can be more accurate than an anechoic chamber. In a nutshell, the measurements show the actual sound coming out of the speaker independent of the room.
Reference axis is the tweeter.
Klipsch RP600M With GR Research Mods Measurements
I am going to contrast the stock version with the new. The former will always be on the left. Let's start with the all important spin/frequency response:
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The stock version is fatally flawed in crossover region with that large hole. GR Mod uses a first order (?) filter to make the two drivers roll off slower and thereby, filling that whole. This gives us a much more flat on-axis response. There is a cost though in sensitivity which drops by 3 or so dB.
We can see the correction better in near-field measurement:
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There were to port/cabinet resonances in the stock version which are gone now. Whether this is due to padding being different, I can't tell. But it is certainly welcome.
The fix naturally improves the early window response:
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And with it, predicted in-room response:
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So very good job there. Let's now look at distortion. This was tricky as I had to match levels. Doing so with speakers of different response is non-trivial but I got close:
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At 86 dBSPL above, it is hard to see much of a difference. Going up to 96 dB gives us more data:
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Stock unit has that broad distortion hump. That is much reduced with the mod but now there is a sharp resonance. There is some reduction of distortion at the far side of the spectrum with the mod.
Company makes a lot of hay out of CSD waterfalls so let's look at that:
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Seems like some reduction in resonances. The peak in the stock speaker around 800 Hz is gone which results in less ringing there.
Impedance and phase plots also show similar improvement:
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Impedance is also brought up a bit which is nice.
I was interested to see the impact on directivity. Alas, my Klippel software is no longer compatible with old files to view/generate them.

I have to find out why so here are the data for the modded one:
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We see controlled directivity courtesy of that waveguide although the tweeter does start to beam/narrow. Even prior, we have rather narrow angle which likely helped with extra energy/sensitivity in that region.
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Vertically we have the standard issues with 2-way design so stay at tweeter height:
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Listening Tests
I started listening to the stock RP-600M and immediately noticed its brightness and lack of spectrum in mid frequencies. I switched it out for the modded version and improvement was substantial. The sound was tonally very balanced now. I thought the highs were a little unnatural so switched out the speaker for Revel M105. There was too little bass in that smaller speaker to compete so I put it aside and put on the Revel M15. The Revel projected a much larger/diffused sound which I much preferred to the RP-600M. I switched back to it and the vocals seemed to shrink to the middle of the RP-600M cone. This is backed by the directivity plot (which I had not seen at the time).
The dynamics were a bit more limited with the mod due to its lower sensitivity. And by this I mean it started to get distorted a bit less than the stock speaker did. Impact was in bass frequencies which became progressively ugly. That said, this was happening at fairly high playback level and with one speaker. So as a practical matter it should be fine if you have enough amplification power.
Conclusions
The flaw in stock RP-600M is so obvious and so is the solution. It was reassuring to see GR Research correcting the response and smartly using lower order filters as to keep the cost down. The difference is dramatic. I can't listen to the stock version. But with the mod, the combination was definitely a contender. You lose some sensitivity so better have a good sized amplifier. The narrow directivity is not to my taste but may be to yours.
Overall, this is a job well done by GR Research and I am going to recommend it to owners if they are not inclined to use my software EQ fix.
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