I would start by ensuring you fully understand your goals here. You said your main issue is with dialogue, but you are entertaining a full 5-channel DT package based on a salesperson's offer. Do you really want to upgrade your entire audio environment, or just get crisper dialogue? It is true that you should match your fronts across the entire soundstage - meaning that the left, center and right should at least be from the same speaker brand, if not the same speaker family (i.e. don't use your existing Onkyo's for left/right and then buy a DT center), but that isn't as critical for the surrounds - if you even want surround speakers.
If you want to upgrade your entire speaker set, and you have set $2K as your budget, then as many have suggested here, go audition a few and decide what you like.