I have a Carvin all tube amp with a Celestion speaker circa 198?. I would like to get something a little more mellower I play rock and roll, rockabilly, Jazz, blues, not metal. I have a epiphone less paul type guitar – not quite as good.
Yes yes speaker!

Wow, nobody really read your question. You just want a speaker, right? Look at carvins web site – they have quite a few options. A higher watt speaker will not "break-up" as easily so may sound more mellow. A single 75 to 50 watt speaker should work fine. Peavey also makes good replacement speakers. Celestions tend to distort easier unless you get the higher watt models. Also, double check the ohm rating of the original – it might be a 4 ohm – so if you replace it with an 8 ohm you’ll lose some output, but gain headroom.