I have been using a 20 foot run of GR8 in an Audio Research SP9 MKII/VT60, Ah! Njoe Tjoeb, Vandersteen 2ce Signature system and have been pleased.
Cobalt was a $25 a foot when new. It is a hyperlitz design that used a combination of functially perfect copper (FPC) and FPC-6 copper. The FPC-6 copper was AQ's best at the time. Cobalt was the next step up from the highly aclaimed Midnight. Gr8 is newer but has always been a budget cable. I believe it uses only long grain copper, which is not as smooth on the highs as either of the FPC's or the newer generation's perfect surface copper (PSC).
I have two 12.5 ft runs of Cobalt I am currently using to bi-wire. I am considering selling one or both of these as I am always looking to play around with wires. I also have a 10 ft pair of AQ Crystal internally biwired. It uses FPC copper for the mid/highs and LGC for the bass. It was the next step up from the also good Indigo cable. IMHO this is one of the best used values out there. If interested in any of these, check out my ad's under the cables.
That said, cables are very system dependent. For example, I swore by Homegrown Audio Silver interconnects until I changed speakers from Spica's to B&W 802. Then they were to bright. I changed to Audioquest and Synergistic Research ic's. I personally have found the AQ's speaker cables to work well with a number of equipment combinations.