This is something that I am going to face in the near future too, as a SB owner.
in my current address, I have severe room issues, that cannot be treated acoustically without ruining asthetics, and I simply don't want to digitally EQ.
moving very soon, will address the DAC issue.
First thing - don't dismiss the DAC in the SB. In my awful room (!), it sounded less 'full' than an audio synthesis DAC, less 'valvey' than a California Audiolabs valve dac, but neither better nor worse than either.
I am hoping that in a great listening acoustic space differences will become more apparent. It is difficult (nay impossible) to hear performance issues in such a compromised space.
On the ''hot list'' for me to audition are -
1) A simple NOS Dac.
2) Prometheus do a nice DAC, cheap too. Definately worth a look this one.
3) DIY hifi supply's 'SATCH' DAC - also worth serious consideration
4) Twisted Pear do a 'Buffalo' DAC - which I am going to bully Paul into bringing to my house

- and reward him with tea and biscuits! haha.
Option 1 - NOS - Ebay, how cheap are they!?!?
Option 2 - This is very cheap for the reputed performance. Check out the reviews.
Option 3 - it's DIY but they build at a price.
Option 4 - it's get your soldering iron out time.
For me - I think it's going to be the Prometheus DAC or the Twisted Pear Buffalo DAC....or maybe Option 5, which is a lovely DAC a friend of mine is bringing into production soon - very, very promising.
So, yeah audition carefully - and again I reckon the standard SB DAC is really actually pretty blooody good.