I have an onkyo sr 705 connected to Polk rti a5 fronts, csi3 center, rti6 surrounds and Velodyne vx10 sub. Considering a csi a4 as center. Will this make a considerable difference? Is it worth the upgrade?
I think the CSi3 and CSiA4 are pretty similar. The CSiA4 is the replacement for the CSi3 center. Same size drivers and Polk list the specs as the same.
The CSiA4 might be a little better match for the RTiA5 towers but I bet the two centers are pretty close.
A considerable upgrade would be the CSi5 or CSiA6. Then you'd have the same 6.5" drivers as your RTiA5.
Based on my limited knowledge, I would concur with afrogt's comment. The CSi3 and CSiA4 are virtually the same in specs. If one were a sceptic, one might even think that polk was just rebranding and changing the shape just to make more money!!!
I just went through the same scenario that you went through, only I was looking to timbermatch my RTi70s and replace a crappy old Boston Acoustic which was a hand me down from my brother. I ended up buying a used CSiA4 locally as a match to my older RTi70s. I am extremely happy with the CSiA4, and the price ($125) was unbeatable in my opinion for its 10/10 state.
Ideally I was trying to find a used CSi40 but these are hard to come by. Alternatively, I was looking for a CSi5 or CSiA6, but the CSiA6 is too expensive and the CSi5 older models are hard to find as well. So I settled on a great deal for the CSiA4 and will wait for a local CSiA6, CSi5 or CSi40 to come a long for the right price, I am in no hurry.
My opinion: don't bother unless you are upgrading to a CSiA6 or even a CSi5. But be warned, those centres are beasts!