I have a Vox AC 15 C1 with Celestion Greenback. This speaker is IMO too harsh.
Best WGS (ceramic) replacement speaker?
Best: Blackhawk
Best Ceramic: Reaper ... for sure!
So, why the Reaper? I can't afford alnico, but I feel the greenback is harsh as well...
The Reaper is just a really big, fat syrupy speaker ... that still retains a bunch of articulation & touch-sensitivity, especially in the non-HP version, which is wound on an "old-fashioned" paper former.
In this case ... it's really the AMP that's causing most of the problem ... those things are just REAL spiky in the (harsh) upper-mids ... and so they need a speaker Rx!
Ya know ... giving it a little more thought ... I'd just bet the (ultra-warm) smooth-cone G12 would really be perfect in that amp. I've never tried it ... so if someone does, PLEASE come back & post here in this thread!!