I definitely think the ET65 will help you retian the sparkle and definition, but also keep you out of the "too bright" area.  It's a very musical speaker that blends in well in most amps.  It would be my first choice. 

I have to second Daniel's recommendation.  Both he and Vaughn suggested the ET65's for my application in my 2x12 JCM800 50w combo.  I just installed them this past Saturday and I am very happy with them.  They have a deep, full and tight bottom end, a wide/not over-bearing midrange and a smooth high end with a nice sparkle.  They sound great with clean, overdriven and high-gain settings and they're not even completely broken in yet!  Read my post on the forum.

Thanks - Would you say the et's are the "best" option or should I consider something else?

I'm becomming more of a fan every day.  The only applications where I would steer folks away from the ET 65 would be if 1. their amp is already overly dark, 2. they play very aggressive/heavy music, or 3. they are looking for a distinctly brittish flavored speaker (think GB or Vint 30).  The MusicMan amps are generally a bit bright (Hey we are talking Leo Fender here), and maybe a bit stiff (Leo was going after a lot of clean headroom ... and so, I would say yes, if it were me, I'd choose the ET65 for that amp.

Thanks folks - It's good to know that sometimes I actually can make the right choice. From the clips and description of the ET's it really seemed as if that would be just what I'm hoping for to balance that amp out.

Thanks again