Do you have a sub? Because the KRK series are a little quiet in the low end, that's the reason KRK has it's own sub.
So with the sub it is a great choice although the sub is around 900 euro, without the sub I would say take something else.
The Saffire isnt an amp, it's would sound really quiet without one and pretty much useless. Passive need an amp that has the relevant ratings to get the best out of them.. Personally its all a bit dated now, Active is the way to go.
i actually downgraded from krk 6s to 5s. always get active, i never even heard of passives. the 6s are so freaking huge, they look like a mechs foot. i have to admit the 5s do make you feel smaller, like in the dick department. but soundwise they are perfectly good to make music on in a smallish studio, which is why i got the 5s instead, i got a combined attic/washing machine room/studio bout as big as a shoebox.
imho they go deep enough to work with them without using a sub. of course it also depends on your room. if you've got a huge room and want to hear even the lowest frequencies, the 6" won't probably be enough. but for the lowest frequencies you can always just use a pair of headphones (with lowest i mean <50Hz)
i'm like logikz in a very small room (~ 3x3m) and it's nearly too much bass from the 6" for it. when i got them i was really surprised, because i didn't expect such a lot of low-end. i even thought about replacing them with the 5", because i thought it was too much for my room to handle. but now that i got used to it and set them up differently i really enjoy working with them although i need some acoustic treatment for my room. for me and my room it's more than enough bass
On a lighter note, if you are suffering with no good sub bass monitoring equipment, you can always bring your bass elements up by an octave - to see how loud they are there. (providing you havent used an EQ to boost) you should be able to get some accurate level monitoring. Just make sure that you buy monitors that have a flat frequency range. Sort the sub bass levels out then bring it back in to the sub freq territory. Its not the best solution by any means, but it'll suffice till you rig up a monster system.
what about the Alesis M1 mk2s?
there like 240 -260 depending on were you go, 6.5 inch woofer. and there pretty nice really, theyve got a pair in the mian studio at my college, as flat a range as your gonna get at this price/size plus, loadsa people have good shit to say about em!