The reviews on the HD600/4 are very mixed over the web. Similar to PDX in that it is another D-class amplifier that can be used for duty other than just subs. I've been on a few audio forums and the consensus seems to be that the Slash V2 series is much better in terms of SQ. Maybe a 300/4v2 could be considered? These amps also take balanced inputs so you could use the same connectors as your PDX amp.
Originally Posted by jonywalker
Fine, it seems to me I have to use a LC6 too. Maybe the LC6 could be configured to set very low input sensitivity, as I noticed the noise output of HU is constant, nearly independent to the volume adjustment of HU. Or, sell the PDX and buy JL HD600/4 since it has the same high level input as my JL slash.