It is best not to mix types of OP transistors - if you change one, you should change the opposite (PNP or NPN) one that makes up that pair. Then, because the AU-X1 uses multiple OP transistors in parallel, to avoid unbalancing the other 4 in that channel (3xNPN & 3xPNP per channel) you should change all of those as well.
And although it doesn't matter quite as much, with a statement piece amplifier like the AU-X1 you should really change all the transistors in the other channel too.
There are measures taken to 'even out' the potential differences in even OP transistors of exactly the same type - these are the presence of emitter resistors, and also the resistors in series with the bases of the OP transistors. Don't even dream about altering the values of those to account for the use of different transistors.