let me know about rivera knucklehead tre55 head

[PS412]

I have never played one, or knowingly heard one ( other than youtube ) but there is one I may have a chance to purchase for a reasonable price.
What are your thoughts? Less interested in clean much more interested in the gain channel and it's usefulness and different characteristics.
I have a couple of other gainer amps and wondering if I probably already have this covered. ( JVM 210h, Triple Crown 100, Prosonic, Amp1 Irridium. )
Thanks