Sunn PL-20 Beta Preamplifier For Sale. We're quite knowledgeable about the market for most items we sell, but we didn't know much about this one's value, so we searched the web a bit. There doesn't seem to be much information about this model online other than a few posts stating how rare and desirable it is. One such post referred to it as "the Holy Grail". Ours has been professionally serviced and is being professionally sold as well and is working great and will be packed properly so it gets to its new home safely. We paid our tech to fully service it. He recapped the power supply in an effort to extend its life and improve performance, cleaned all pots and connections, and tested this unit thoroughly to make it better than other units out there. If there even are any others out there.