A friend purchased two Jan Phillips 5751 tubes for his guitar amp (which he says has worked well), which were advertised as 12AX7 replacements.  He's offered one to me to place in my Z-Box.  

I wanted to know if that is a suitable replacement, or would installing it kill something?

In regards to gain, I use a Monica 2 DAC with I/V stage which has a lower output than a normal CD player, therefore more gain (or at least less lost gain) than from the 12AT7 I am using would be good.

Simon