As I'm looking into playing out - with acoustic guitars - I thought it would be worth my while to see how my Spider IV 75 might work for this.
Using headphones, I plugged in my Ibanez 12 string acoustic/electric, fiddled with the guitar tone controls, then put the amp on Clean Green, dialed the drive down to '0' and was pleasantly surprised to hear a nice clear sound.
Today, I tried it with the speaker, and cranked it up, too! I had to adjust the tone again, on guitar and amp, to allow for the loudness, and still got a fairly clear sound even at full volume both on CLean Green and Clean Red. It wasn't as loud as I had hoped, but I dialed in just a bit of drive and the volume went WAY up without much noticeable distortion.
As I had anticipated having to plug acoustics (and keyboard, when used) through a mixer and into the MP3/CD input, this is all good!
Both the Red and Green clean are great places to start for clean tones. I prefer the voicing of the Green, but both will fill your needs when you have to have a "clean" sound. And yes, it is helpful to add some drive to those models to get a bit of gain out of the amp and more "oomph" to the sound.
...but be careful of adding to much gain to the Green clean channel, because it does start to drive from about 75% to 100%! (whereas it almost impossible to make the Red channel distort.)
The Green clean (Hiwatt model?) sound fantastic with an electric guitar at 100% drive...very Pete Townshend!