Ill probably have around 400 as soon as i sell my 15 in compound miter saw it just sets on my porch doing nothing. Or would it be better to buy rp450c center channel next. Im trying over time to get 5.1 going. Might take awhile.
First, be happy that you now have a pretty nice sounding system. I suspect the change of speakers is by far the most significant improvement you will see. A good sub or center will likely improve the sound, but by a lesser extent.
I would say the decision between sub vs center would mainly be dictated by your application.
How much music vs Home Theater do you intend this system for?
I am probably 75/25 music vs TV, but when I eliminate TV shows that don't have real attention to sound (like the news or comedy shows like Jimmy Fallon - where your 280's are more than adequate) I am more like 95%/5% music/HT!
Almost all music is recorded in stereo and you are best off not using any of the synthesized multichannel features of your AVR! Thus, for someone like me (95%/5%), a center speaker offers very little, since it adds nothing to music. A sub, on the other hand will round out the bottom end of my stereo music, so is much more important
to me.
I should add that there are some good music recordings made in 5.1 (or other multi-channel formats). Some of them are quite well done by people who really embraced the technology, but by and large, they are novelties - I say that because of their rarity and extra cost - I don't intend to discount the effort some groups have invested to produce a quality 5.1 recording. Others simply use the extra channels mainly for ambient effects, and then there are a few that should not have attempted 5.1 without more forethought!
If you listen to lots of TV and movies, then the center may be of greater worth (although many would still suggest the sub as more relevant). Just listen to your system as it currently is, how well is the center image maintained (without the center speaker)? Close your eyes and image a center speaker, is a fair amount of sound coming from this Phantom center? Some speakers do this better than other and I suspect the 280's do it well. Do you have difficulty hearing dialog with your 280's? If so a center may be very worthwhile for you since you can raise the volume specifically on the center which almost always carries the majority of the dialog!
I went 5-6 years without a center, even as I went from 2.2 to 4.2 (surround without a center). I finally added a center and really don't feel it made a big difference in my sound. But for others, adding a center was a major improvement, so check the above aspects with your current speakers to get a feel for what you might gain!