I don't know what you mean by "have trouble pushing them". My dad's Marshall Class5 is a 5W single-end class A amp and it has no trouble roaring through my 240W cab!
If you're concerned that you'll blow the speakers, to determine the wattage of the cab, you take the power of the lowest-rated speaker and multiply that by 4 - so a cab loaded with 2x60W speakers and 2x25W speakers has a power handling of 100W. So 90W into 100W is safe enough, unless you intend to run the amp at full tilt, then you're risking it a little.
There's sort of a rule of thumb that your cab should be at least 100% the wattage of your head, but that's just playing it safe. I know guys pumping 100W into their 1x12" 25W cab and they're fine if they keep the Master down.
The lowest rated guitar speaker I've seen so far is the Celestion Greenback, which is 25W, a cab of those is 100W. Mills Acoustics use V30s though, which are 4x60W=240W, so you're safe. If you intend to run a full stack, don't even worry about it, just make sure you get the impendances right and start rockin' 8)
I'm no expert, that's just how I understand things, so if I'm wrong at some point, please correct me.