SB Z doesn't really have a headphone amp, not in the same way the ZxR does and STX/II at least. I'm not talking about quality, just what might be referred to as a dedicated headphone amp.
Creative claimed when the Z was released that it had a headphone amp that could drive 600 Ohm headphones. If you look at the Z specs now, they have removed the claim that it can drive 600 Ohm headphones and there is no mention that it has a headphone amp.
The Z has a weirdly high output impedance of 22 Ohms though, which would be fine for higher impedance headphones, except it's output is not enough for such headphones. :/
Looking at the 1540 specs they look easy to drive at 46 Ohm. I doubt they are weirdly harder to drive than the impedance suggests, like AKG K702 for example; but the SB Z is not the best source if you have high quality headphones, so I'm certain the headphones can benefit from something better.
I wouldn't look at something like the N22, unless you need an amplifier for passive speakers as well. Buying that because it has a headphone socket for just headphone use would not be good idea.
Do you use the headphones for gaming? If yes and you want the whole positional sound thing provided by SBX prostudio, then you'd be best getting something like an SMSL M6 and connecting that to the SB Z optical out, which can then pass on SBX prostudio to the external DAC/amp.