I've A/B'd a ceramic vs Al Ni Co speaker before (Weber Blue Dogs) and found a warmer (less bright) sound with a Al Ni Co magnet (all other factors being equal) than a ceramic magnet.
I recognize I'm wading into subjective territory here, but I'm bias toward an Al Ni Co magnet.
I've tried about 30 speakers in my blackface Deluxe Reverb and have the EJ in there now. A blackface Vibrolux Reverb with original Jensen's is a wonderful jazz tone.
Pre-Rola Celestion G12H30s are up there as some of the best all-around speakers - known for Marshall's but their clean sound is warm and woody. It is impossible to say that "alnicos are warmer" or "ceramics are brighter".
I have never had experience with Peavey Delta Blues nor the Mesa Mark V 35 so I cannot give You a straight answer – if there is such. I like the Ipswich made Tayden ACE 25, ACE 50 Alnico.
So my second question is: what is it You do not like in their stock speakers? Don't have it meself but I know someone who does in a Tweed circuit. I find it truer in tone than Celestion's own copies of its traditional Blue.
(Oh-la-la, first smiley I use in my forum post in internet ever! I have never had experience with Peavey Delta Blues nor the Mesa Mark V 35 so I cannot give You a straight answer – if there is such. The Peavey Delta Blues has a 15" Weber ceramic Blue Dog speaker.
That "should" give you a bit more headroom before break up.
Scumback Scumnico - little more low-mids than the Gold but otherwise similar ASW - most balanced of the 3, imo - way more expensive as well My favorite for clarity would be a JBL D120. Old 60s Jensen P12Ns are pretty good but can be a little sterile at low volumes.
Works nice with fat treble strings as the aluminum cone kind of shoots out the higher frequency notes. Great when cranked and crunching, but can lack clarity for clean jazz, especially with big strings. Old P12Qs are probably too low watt to handle loud cleans.
There is nothing wrong with this speaker necessarily. Say 'terveiset sinivalkoisesta Suomesta' to Your relatives! I think there many variables in speaker design that have a stronger influence over tone than Al Ni Co vs. I think Al Ni Co will compress more than ceramic which might also affect tone at high volume levels.
I'm compulsive about changing things when I should just play my guitar and not be concerned. Say 'terveiset sinivalkoisesta Suomesta' to Your relatives! There is three Celestion Golds for sale in The Gear Page Buy And Sell pages. This is gut feel based more on reading than experiment.