I may need Nomex knickers after this one: :rolleyes:
Celestion speakers are generally designed to 'interpret' what comes out of an amp. Different models have differently-shaped frequency responses, and different levels of compression and breakup.
All of which can allow a player to get a particular kind of sound out of their [u]tube[/u] amp. Which a Flextone II is just modelling wannabe.
What you need, is a very wide, flat, frequecy response, with no distorting breakup. The same kind of speaker you would choose for a stereo system. Any speaker that adds its own character to the sound is going to reduce the accuracy of all those amp models that you paid for.
Mind you, your "sounds like its missing something" complaint may simply be due to the fact that you're playing a modelling amp instead of the real thing. (In the opinion of a cranky ol' bastard, of course). :cool:
My recommendation - a Weber 12" California with a ceramic magnet and a paper voice coil dome. It will handle amazing amounts of power without colouring the sound or distorting. I use a 15" Cali in my Ampeg combo, so I can vouch for the design from personal experience. I love what my amp can do, and the California brings it all out clearly, instead of 'interpreting'' it for me.
Don't bother with Celestions until you get one of the Marshall, Vox, or Hiwatt tube amps that those speakers were designed for.