I'm using it on Sup 3 to Sup 4 and it really does give a significant boost to the dialog channel. Yes it is true the higher up you go the tinnier the sound gets. My solution is to run a separate stereo speaker with full frequency content on a fairly low volume, and then balance the ZVOX to bring the dialog out.One thing to note, it appears that if you put on the surround you lose all the dialog boost so I personally am not seeing the benefit of it having surround. I strongly recommend this be used in conjunction with some other full range speaker system which is turned down fairly low so you get the strong dialog without the tininess. That being said, on its own without being too picky about fidelity, you can make the dialog incredibly loud compared to the sound effects so in that sense it does its job very well.
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