I recently got an SB Touch which iI had a full set of analog and digital mods done to, and will be building a very nice, superregulated 5v power supply for it, and I expect that it will be a superb source for our living room system (connected via ethernet to a NAS in the basement.)
Several years ago I had a fully decked out SB3 with high end power supply, dc umbilical, etc. and it was at that time the best digital I ever had. The modified SB Touch with a good linear ps and the software tweaks Shawn mentioned above should easily out do the sb3.
I don't think it will beat the mach2 mini -> ap-2 -> metrum dac combo, but I'm keeping an open mind.
There is also a very inexpensive analog mod for the Touch that Bolder cable can do, which basically removes the cruddy electrolytic coupling caps on the dac output and replaces them with a piece of wire, and along with the soundcheck software tweaks, can take the basic touch up several notches.
For another, totally different approach to a wireless dac setup that is not compromised by being wireless, take a look at the Audioengine D2 dac -- if you use the analog outs on the receiver box (instead of optical spdif) there is a volume control on the transmitter box that does not interfere with the digital music. BTW, this does not use the standard wifi 802.x wireless protocol, but a proprietary one just for audio and suffers none of the same issues as the regular wifi audio transmission. Oh, and you can connect up to 3 receivers to oe transmitter. So basically, you'd plug the D2 transmitter into your laptop or htpc, connect the receiver to the amp and use some sort of remote app to control the music playback from the htpc or just the keyboard if using a laptop. Transmitter can accept up to 24/192, but it transmits to the receiver at 24/96. This was going to be my alternative to the SB touch but I decided to go with the modified touch instead.
BTW, the audioengine d2 reciever has an optical output, so you can also use it as a wireless usb-spdif converter, which you can plug into any dac with an optical input (bh dac, for example.)
HTH,
Jim