Upgrade Time

I had Mcintosh separates for years, went with a Luxman integrated just last year (I could have gone Hegal and others, but that's another story as to why). I find it sounds way, way better, however to be fair, my Mac was getting quite old. Anyway, lots of pros and cons to going from separates to integrated, less wires, less "what's not working right, easier to operate for the wife/girlfriend/significant other, or all three...There are cons, if you do want to upgrade, then the whole thing goes, rather then just switch out the pre or amp for a better one (although some like the Luxman allow you to bypass the internal amp and use it for a pre only, but I find that silly - you now paid good money for something you'll not use). In theory separates sound better, a $10K amp (which Luxman has in the 900 series) I am sure would sound better then the 590AX II integrated I brought) but that would have double the cost of an integrated once I added in their pre. One other thought, I like to keep things for a long while, I don't have "upgraditist", if I did I would have gone separates again. As always, just an opinion based on my personal experience

Since it seems you would be fine upgrading the amp now and waiting to upgrade the the pre later, that sounds like the route for you. 

It would be a shame to drop $8-$10K on a power amp and pair it w/the Cambridge for the next couple of years. While I agree with all the caveats posted above(space, extra cables, etc.) separates are generally a wiser investment. 
Are you talking new or used gear? Either way, I'd rather see you upgrade both amp & pre. For example, get a Backert Labs Rhumba for $4K and your fav $4-6K power amp that pair well w/the Sonus Fabers and you'll have a blast instead of waiting 2 more years to hear what your new super-amp can do. Cheers,
Spencer 

What sources do you use other than the streamer portion of the Cambridge pre? 

Just one thought, start the upgrade path from the source. Separates will allow for this. Replace the Cambridge full function preamp with new DAC and line level Preamp. Providing the Cambridge amp it is driving your speakers satisfactorily use it till it can be replaced. The search for 'the' amp for your speakers can continue. Upgrades in the front end of your system will be appreciated till that purchase is made.