Thanks for your very positive reply but first I have an update.

I've just completed a second session with the Celestion speakers and once again compared the new G12-Neo to my two familiar speakers.

Yes, the G12-Neo still sounded different to the others and yes, This gave me the confidence to turn the amp volume up to 5 as I'd noticed that as well as being tonally different the new G12-Neo was also "quieter" than the other two speaker on the same settings. I know this comes under the heading of sensitivity (97dB versus 100dB) but to me the G12-65 Heritage (97dB) sounds louder than the G12-Neo (97dB).

Anyway, with a little more push from the small amp the G12-Neo began to respond as I'd expected. The characteristic compression kicked in and the single notes that felt too 'lightweight' before were now clear and chimey. I added some overdrive and it handled the harmonics just as I thought a Creamback should ... my confidence is restored. The G12-Neo has a different tonal character to the other two speakers and needs a bit of time to get used to ... but that's how it's supposed to be. This is a new design and I'm now looking forward to using the G12-Neo with both my amps and finding out what it can do.

Thanks once again for the great customer service and I'll be in touch again soon, once I've worked out which speakers & cabinets work best with each amp.

(I think I might have some ex-demo WGS speakers for sale on eBay in the meantime)