I have been a GK head guy for some years. Got away from tube sound some time back, and wanted to go back. Really like the bi-amp capability with internal crossover in power amp and separate volume for woofer and tweeter my Gk head (1001RB-II) gives me.
To get that tubey smooth sound back, I tried several tube preamps (Ampeg, Ashdown) and decided on the Fender.
I have been very happy with it. I run the fullrange output of the preamp into the FX return of my GK head and have the best of both worlds. I run a EBS Unichorus configured into the FX send/return on the rear of the preamp, and control with the preamp footswitch.
The preamp has an incredible range of sounds from deep warm bluesey low end to poppy high end for slap. The tube overdrive is just that, it adds a footswitchable overdrive moderate distortion to the already warm tube sound for a great blues and rock sound. The notch (they call it Vari-Q) is very cool. It is also footswitch controlled, and gives you a quick EQ change for Johnny Winter-sounding trebeley bass parts. Something I do alot.
All in all, though pricey, it is a great addition to my rack, and find it meets all my expectations of function, price, and size.