System upgrade, need advice, source or speakers?

Hi,

I've had my system for a few years and have been thinking about tweaking it. Right now it consists of:

Speakers: Zu Definition MKII
Amp: Tom Evans Linear A
DAC/Pre: PS Audio Perfectwave DAC

All of my music is digital (flac) and the only input I use is USB hooked up to a mac mini.

For the longest time I've been thinking about swapping out my PS Audio DAC for something else. 1) I only need the USB input so I don't need the bridge etc that the Perfectwave offers. 2) I've always had the sneaking suspicion (for no particular reason) that the DAC was the limiting factor in my system.

I started looking into higher end DACs like the Lampizator Level 5 and the Calyx Femto.

But now I'm wondering, would I be better off keeping the Perfectwave (or swapping it for something like the NAD M51 or Berkeley Audio DAC) and upgrading my Def MKIIs to MKIIIs.). The price for either option is roughly the same.

Any advice would be appreciated.