Well, use of an external selector is not a lot different than having that facility built in to something. There will be an extra set of interconnecting cables involved, of course, and opinions may vary widely on the "best" switches and connectors. Unfortunately, in this hobby, interconnects, switches, and connectors can very easily exceed the cost of many amplifiers!
I cannot speak much to the modified Quickie, since I don't own one, except to say that it was initially designed as a preamp only, whereas both the SEX and Crack were designed with headphones in mind. With the SEX, of course, there is considerably more power and the added ability to drive efficient speakers.