Acoustic guitar pickups LOVE tubes in a big big way, but I've found that they don't generally like the EQ voicing in a Fender amp. Seems better to use some type of flat response tube buffer, and then use a flat response EQ/preamp to tailor the high/low balance, maybe notch out that mid-bass feedback frequency, and then maybe work with a possible barky midrange factor. Acoustic preamps are good about addressing those EQ needs, but most of them sound like crap without the help of a tube somewhere because piezo pickups only become more harsh and plasticky sounding going into transistor devices. The signal from a piezo pickup is wildly unwieldy, the output impedance from a piezo is a very inconsistent and unstable thing, and that's why so often amplified acoustic guitars have that gritty, glassy, harsh haze that irritates the ears and veils the natural guitar sound. And especially with hard strummers. Tubes by their very own nature can handle this fluctuating signal from a piezo far better than FET's or other transistors. A passive piezo pickup directly into a tube sounds more like a microphone that many would believe. The tube's ability to track that signal actually gives a much cleaner and lower distortion sound than solid state devices. That crispy harshness we hear from amplified acoustic guitars is basically distortion. Tubes remove much of that stuff and you're left with a surprisingly natural, wood and string sound. Still kind of weird because the pickup is located at one place on the guitar unlike a mic out front that sees the whole instrument, but the tone character thru the tube is SO much more pleasant than with a transistor.
Even if a guitar has active electronics in it, a tube immediately after that stage helps tremendously because that crazy, spiky, high frequency garbage only generates more and more distortion as it passes thru consecutive transistors stages. If a tube is right there early in the path, the whole sound is sweetened and cleaned up, and then any following transistor preamp stages are able to simply EQ and drive without adding more harshness.
I've definitely tried using the SMS or similar tube circuits for acoustic guitars and the result was just kind of weird. The tube factor is nice, the but tone shaping is wrong. The Fender EQ is far from flat and to me is really for electric basses, steel guitars and electric guitars. Oh, and Rhodes pianos too.
... and it's just like any other day that's ever been...