For guitars and some kind of vocals, drums in general I like my api A2d a lot, I'm in search for a good channel preamp to use with clean voc that have a different caracter than api...i'm intrigued also by the new 4 channel 710 ua preamp but I don't know how is the best choice.
If you already have the API, the UA will probably be a really good companion
Love mine, never had any issue. Until June 30th you could also buy a 6176 and get a free UAD-2 quad with it ($1499 value), hard to argue with this!!!