Your ears are the better judge of whether or not the R700s or R900s are better if you have the chance to listen to the both. To find people that have heard them, you might try AVS's KEF owners thread
Kef rates their -3db points at 42hz and 40hz, so the differences in bass extension are insignificant. You would listen for whether or not the larger drivers provide more textured bass response. Also, the larger drivers in the R900s might have better dynamics if you like to listen to speakers especially loud and/or in a large room.
As for a sub, well, depends on the content you listen to, placement options, and how big the room is:
1) Most music would do quite well with that kind of low end response, except for some bass heavy genres. Movies and gaming will benefit from the lower extension down to 20hz that a really good sub can provide.
2) Bass response is affected by room size. If you have an especially large space to fill, the speakers may not do the job.
3) Speakers have to be placed where they are best able to produce upper bass, midrange, and treble for the listening position. That is often not the best placement for producing bass to fill the room.
If bass is really important to you, then R300s with dual subs could be the better way to go. Dual subwoofers can provide a smoother frequency response though a wider listening area. If you can, listen to the R300s with a good sub, and then listen to the R700s/R900s without. Maybe you'll like what a good sub can do over towers. A lot of people do.
For subs, I recommend looking at offerings by SVS
. These companies offer much better values than traditional speaker company subs that you would find through a brick and mortar store. Just keep in mind that you need enough sub for the room space and how loud you listen, so a small sub may not work well in a larger room.
Finally, since you are just getting into this, learn about room acoustics
. Being smart about placement and using room treatments can easily make more difference in sound quality than say going from the Kef Q to the Kef R line. Room acoustics almost always negatively affect SQ, and particularly subs generally need some kind of EQ to achieve a good, linear response.Edited by cel4145 - 2/7/14 at 5:08pm