JCM900 Combo speaker Q's

I just bought a JCM900 50Watt 2 12 combo off ebay. It works great, but I was wondering how it would sound if I swapped the G12T 75Watt Celestions with the set of G12M 25 Watt Green Backs I have? If I wired the two 25 watt 8 ohm'ers in series they would handle 50 watts at 16 Ohms.
And have any of you mucked about with swapping tubes in a JCM900? Would you? Why?

Mike

If the amp is rated to work on 16 ohms, you can give it a try ... Just don't mess with the wrong impedence.

What tubes ? I always thought the jcm900 was a solid state amp .

That's funny.
http://www.theguitarweb.com/marshall2.htm

Wait a min , I know there was a tube version of that amp ... But the original design was Solid State , or hybrid (tube preamp and transistor power amp) I think , yeah ?

Hmmm. Well, the one I have looks like the one in the link, except mine's in a combo box. If you scroll down the page on that link, you'll see a pic with it's top off showing the tubes. Mine was advertised as "All Tube".