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I've known of these for awhile, as they are a non-electrolytic capacitor in large values at fairly reasonable pricing via NOS or used.
Being that a capacitively loaded sealed box involves larger capacitances than the average audio or industrial grade poly cap unless paralleling for value, these Unlytic types fill that niche with reliable long term use involved. Hopefully better sonics also will result. Through Don Radick's recent experiments, Tony Gee's Black Box, and my PC project; i've come to the conclusion that 500-600uF tends to be close to the sweet spot for 8 ohm loads. I also used that value in the EMP highpass/warmth notch.
It just so happens ebay had a listing for 600UF/300VDC Unlytics. The price was right, so i grabbed some of them. Be aware the shipping was not cheap. I'll report my findings as things progress....