As the owner of several G-flex cabs over the years, Ive experimented with different speaker configurations. The design of the cabinet makes this a great nu-metal choice, although Ive used it mainly for hard rock/metal, but its recessed mids nature (and overblown lows) makes it sound a bit un-traditional.
The stock cab is very midrange challenged (scooped in the mids). Ive felt they have a hard time cutting thru a mix & want to occupy some of the bass's territory.
A V-30 set will thicken up the mids pretty well, and still keep your bottom end.
My favorite replacement speaker (for the Genz cab), and the ones I currently have in my G-flex cab is a set of 100 watt Carvin British series 12" speakers. These speakers were in a Carvin cab I bought about 5 years ago. I bought it on a lark. Found it when I visited Vegas & decided to pick it up & try out with my Mesa Roadking & XXX amps. I ended up not using it too much because it was MID-city. Partially because of the cab design, but mostly just the drivers. I think of these drivers as Vintage 30 with a HUGE mid injection.
About two years ago, I decided to try them in my G-flex.
Perfect combination.
Nice, tight low end. Smooth mids, silky highs. Huge sounding.
The midrange challenged G-flex cab & the mid heavy Carvin speakers cancelled each other out, and are a perfect match.
Just my opinion.
BTW I havent played through any of my cabs the entire time Ive owned my Ultra. Close to two years now. Just the occasional use of my Black Pearl combo.
