As a guitar player who doesn't have any stock part of the rig and customizes everything I can tell that Celestion Alnico speaker is best upgrade to my tone I have ever done. Not the amp, not pickups, not wiring, not isolated power supply, not 4mm main lead with EMI/RFI filters with audio-grade fuse and hifi power plug, not power conditioner, but amp speaker. This is where 90% of your guitar tone is born.
I refret my guitars, replace pickups, put treble bleed together with 50s wiring, upgrade every part of the rig, put cascade of overdrive and booster with high quality interconnects. Every step helps and I can hear clear improvement in the tone and noise floor reduction but stock amp speaker ruins most of such improvements. The weakest link in rig's chain was Boss Katana 50 speaker, overrated amplifier with stock speaker but top quality amplifier when speaker is upgraded. Have you tried budget amp with high quality speaker? It changes everything.
The lesson learnt, stock ceramic magnet ruins guitar tone. Now Alnico magnet speaker restored all the potential and customizations I have done, every little component plays its role in overall sound and it sounds fantastic now.
Now Boss Katana 50 sounds better than Artist I used in the guitar store but one significant downside is now I hear almost no treble. Is it good? Initially I was dissapointed because I compared it to stock speaker sound. But after two weeks of using this speaker I realized the guitar tone cuts through the mix much better now. I can compensate loss of treble with guitar pedals eq knob if I need to.
Customization to my guitar rig isn't over, many parts will change but Celestion Ruby will stay 100%, highly recommended.