I'm now a firm believer in the dirty strings theory (or science). This Thursday will be 1 year with the same set of strings on my J-45, and I've really grown fond of the sound. They are NOT dead sounding, but nice. I'm a music major and play guitar every day. Since day 1, I've been using the GHS fast fret product to clean the strings. Maybe not EVERY time I play, but most times I play. BUT I've never really wiped them down after using it.
A couple days ago, I finally decided to use a terry cloth towel to wipe the stings down after using the fast fret. The towel didn't show any signs of dirt/filth, but the strings suddenly sounded and felt brand new. The string squeak that faded after the first few weeks of playing was suddenly back along with more top end. I guess the fast fret made sure the strings didn't get TOO dirty, but it didn't really clean them out like the terry cloth. I'm hoping to make a video comparing them to a video when they were first put on. Not sure there will be much of a difference now..... really amazed.