Hey All,
In the process of building a sound system, I have already selected the following components:
Towers: Paradigm Studio 100 v4 (Most probably)
Pre/Pro: Onkyo SC855P
Amp: Emotiva XPA-3
After the above, I only have budget for either a sub or a center channel.
What should I go for first?

(My usage is 50 HT : 50 Music)