Bought mono blocks, now spkr cables too long

Get them cut to desired length as others have said and terminated properly. An 8ft pair is probably more common than a 12 ft so can probably sell the longer portion more easily. 

So maybe its a message to those destined for monos. A mono plus is shorter cables.

It is. The advantage of monoblocks is short speaker cables- so you get more impact and greater resolution without adding brightness.

The advantage of monoblocks is short speaker cables- so you get more impact and greater resolution without adding brightness.

     Agreed!

emergingsoul OP
I’m running a pair of XA 60.8s to the Magnepan 30.7

Just back from hols, keep the 12ft cables you have, as those XA60.8’s "could" be replaced in the future, as their lowish power in a big room to take those speakers may become a problem after a while for you, with those big mulit paneled Maggie 30.7’s.
https://www.magnepan.com/www/images/speakers/listening.30.7.jpg
And then you may then get a bigger poweramp, maybe stereo again, and you need the 12ft cables back.

Cheers George

@noromance with respect to a home audio system there is no length that could be considered too long. Sound and/or data is transmitted instantaneously. That's why you experience no delay when talking on the phone with someone across town or working on laptop sharing data on a zoom meeting with co-workers several states away.....etc. The speed of light is so freaking fast it is hard for most to comprehend. The signal can travel around the earth 7.75 times per second. Bet you can't measure a difference unless the cable has unusually high impedance. Doubtful if under 30 feet, in which case I feel very sorry for your wallet 😬