If the difference is enough to be audible, why not stretch out the session and decide if it's worth investing in?.
Who cares what wins the A or B test. Does anyone listen that way?
If you do determine there are differences between a and b, the test is over. There is no "winner", the only fact is the listener can hear a difference between a and b.
At that point you are free to go and decide which you prefer, a or b.
steve