I have a pair of cabinet flats for sale for the GR Research subwoofer kit including the amplifier cut out for the Apex Sr. 350W amp which Danny recommends. I normally sell these for $150 each but am willing to let these go for $100. These were originally cut out for a customer who ended up backing out so I'm reducing the price to get them out of the shop.
I can also assemble them for a nominal fee if anyone is interested.