I can't recommend enough using the AFD with a switcher like the GCX to make one's setup more versatile. I have an 8 units rack alongside the AFD that has a furman, gcx guitar switcher, a G-major, decimator, some pedals, and most importantly a Randall RM4 preamp. The GCX allows me to send to the AFD's power amp either the AFD's preamp signal or the RM4's. In effect that turns the AFD into a 5 channels amp with minimum hassle (and most of the times I'm happy with a clean channel, a crunch channel and the AFD for overdrive). No need to carry another head and speaker. The AFD is a great amp for such a setup because the 6550 tubes have more headroom than EL34 so it is more neutral when using an external preamp.
This makes for a highly versatile setup. The RM4 is a key part of that sound. I did experiment with other preamps before (mesa boogie studio pre was a great match), but the ability to switch preamp modules according to needs is a clear advantage. Even tough it hasn't been a commercial hit, Randall's modular technology really works, and matched with the AFD it gives me the benefits of a great plug and play head as well as the flexibility of racked technology. I don't have anything to sell here but since for once I have a gear combination that really works, I wanted to share it!