No one actually knows how to lculate what speaker cable they need

Opera Consonance  Cyber 800 monoblocks to Focal 936 Electras connected with Jena lab Trio speaker cables A really nice pure all copper cable.

For the gut with the Bryston 4BSST2 and 24’ long cables:
You are not doing well. The Bryston 4BSST2 http://www.bryston.com/PDF/Manuals/300006[3B4B].pdf have a DF of 500. You need a 3x0 AWG! it may be expensive...
You better get closer, as 8’ and use a single 0 AWG. An other option, is to used two monoblocks and a shoer (3’) of a 4 AWG cable.

For the guy with the  Opera Consonance Cyber 800, even thou you have monoblks, because it is tube (very bad DF) and no data on DF! you may use any cable over 24 AWG thick (home phone line).

Mr. sleepwalker65 your math is wrong. The cable is fighting the power (P) or the current (I). I used Klipsh speakers (8 ohms) with 99dB/w/m SPL with a Caver 250W amp. that played at
0.001W-1W as loud as it should and still a thicker cable improved sound! 
The coin been lost under the Damping Factor (DF) parameter. The higher that is, the thicker the cable you need. It is about controling the speaker, not doing a welding, towing a truck or any other idea you were throwing as you thought it is funny.  

Mr. cakyol
All cooper for electrical use is 99.5% pure. Try to get one with less!
Most cables are using wires as thick as 12-14 AWG because that’s the thicker they can fit into their banana plug (Not a very sintific one).
All qualities they say they give the cables: Cooper purity, use of silver, cryogenic treatment, Bi Wire, Burn In etc’ - has nothing to do with the spec. of a speaker cable. It’s all snake oil mambo jumbo, to cover their and customers ignorance.