Again, you aren't being clear. Squeezebox is both hardware and software.
You can continue to use SB software as you are now. That has nothing directly to do with any SB Touch mods or added devices, such as a USB to SPDIF converter.
A SB Touch whether modded or not, whether hooked up to a Pace-Car or not, will connect/output to your DAC by SPDIF. The SB Touch itself doesn't have digital audio inputs, it's input is from the network. It can connect directly to a USB HD instead of the network, but even in this case your audio output will still be either the anlogue outs or the SPDIF outs. There is also no network AUDIO out on the Touch. The Touch sends and receives data over the network, but it isn't sending or recieving an AUDIO stream this way, just the data contained in your music files (and that isn't isn't an audio stream). When the Touch sends your music files as data, it is turned into audio with whatever audio setup is on the receiving end.
So getting a Touch and hooking it up to a DAC is a separate issue from your setup with the Airport Extreme. I think to do the same thing with SB software, you need to run an application like Squeezeplay on each computer. To do it with hardware (and have the devices synched) you need to add some kind of SB device (Touch or other SB device) connected to each computer.
In the above 2 scenarios, if you are using software, the quality of the playback will be determined by the setup of each individual computer. If you use hardware (additional SB devices)then the quality of playback will be determined by the quality of each SB device and how you use it to produce music (digital or analogue outs, and the rest of your sound playback hardware).
On the other hand, connecting a digital out from your SB Touch to your DAC should greatly improve the quality of playback - but only on that system, not on the other systems in your house. Adding a Pace-Car in would probably also help, but that's an expensive and limited solution in your case. If you got a USB to SPDIF converter like the Off Ramp to feed your DAC, that would certainly sound great, but you'd be bypassing the SB Touch. The connection is Computer USB>Off Ramp>DAC. In this case you are bypassing the SB server software and hardware. (Note:There is probably a way you could install SB server software and Squeezeplay on your computer, setup an OFF Ramp as the default soundcard, and then play music from that computer using Squeezeplay as the player. But that seems a very odd way of setting up a system.)
If general, if you are interested in just getting better computer sound from your main system that the Metrum is part of, then an Off Ramp directly connected to the DAC will probably be your best bet. If you want to continue using the SB Touch in that system b/c you like the interface, then get a Pace-Car and your Touch modded to work with it.
But as your setup is today, the Touch directly connected via SPDIF to the Metrum should work and produce good sound from your main system. If that isn't working, something is wrong with either the Touch or the Metrum or the SPDIF cable. Get that working first and see if you are satisfied with the sound. It may be all you need.
Again, any setup like this with the SB Touch has no direct connection or influence over how your setup using the Airport Extreme sounds.
If you are asking how to improve the sound on the various other computers that are connected by the Airport Extreme to your system, then the answer lies in improving the playback on each of those systems. The Airport Extreme is giving each computer perfect data, but the soundcards, DACs, and speakers built in to those systems aren't up to snuff when converting that data to audio.
And I don't want to be mean, but it does sound like you don't have even a basic understanding of what the SB hardware and software do, or what the difference between an audio stream and a purely data (network) stream is. You should try to get yourself straight on that before spending any more money.