What's a better tweak/upgrade? USB DAC or ERT

Hey everyone,

I've been contemplating spending between $200-400 for some upgrades to my system. While I could always throw that money at cables or something, I really don't think that would be worth it. I was thinking about either trying a USB DAC or a super tweeter (probably the AV123 ERT, but maybe a DIY with a Fountek or LCY). What do you guys think would give the best "bang-for-the-buck".

SPEAKERS: Soliloquy 5.0s with lead filled matching stands
AMP: Bryston b60r
SOURCE: Apple Airport Express streaming Apple lossless format from a MacBook Pro.
CABLES: Acoustic Impact Sonic Wave (a good budget cable in my opinion)
ROOM: 12' x 20'

My next major upgrade will be new speakers; however, I thought maybe I'd tide myself over with something smaller for now. What do you guys think?

Happy Listening!

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Thanks for the responses.

That is a bummer about the USB only being for printing. The vast majority of the time my Macbook is tied down to my external monitor in my home office, so it looks like my best option is to use the aforementioned mini to toslink adapter, and run that to the DAC.

Does anybody have any recommendations for a good adapter for this purpose? I Googled it and looked at a few - most of which looked pretty cheap.