I would tend to agree with tnaughton with regards to using a keyboard amp as this will give you a better representation on the low end. I would steer clear of the Acoustic Guitar amp as I expect you will bottom out the low end, regardless of what pickup you are using. Here is a link to my review of the Marshall S50 that you mentioned:
viewtopic.php?f=4&t=4915Surprisingly good. You might also check out the Acoustic ag120s if you are playing at lower volumes. A nice cabinet and good tonal control with complete separation for each channel.
I have had good luck using a 90w Behringer keyboard amp for practice, you can grab one of these off of craigslist for ~$100 - ~150. Lightweight and decent tone, but no separation with regards to eq, a pretty valid point that tnaughton brings up in his earlier post.
Try them all and sleep on it [grin].