AsRocks M3a770de is a cheap enough board 
theres an open box right now 
http://www.newegg.com/Product/Product.aspx?Item=N82E16813157176R&cm_re=m3a770de-_-13-157-176R-_-Product
but personally id just buy the normal one 
http://www.newegg.com/Product/Product.aspx?Item=N82E16813157176&cm_re=m3a770de-_-13-157-176-_-Product save any potential hassles,
they unlock cores fine but as I said, dont depend on a successful unlock, its a bonus if you can but if you wanted/needed a quad, you should've bought one imo, not sure about warranty voiding, like any manf, they'd probably try and wriggle saying you'd damaged it yourself by attempting to access something that was locked off 
Moto