How can I improve my HT audio?

My current setup (5.1):

L/R: OLD infinity Kappas (6.1's)
center: Definitive Tech 2000 C/L/R
Rear: Definitive BPX
Amp: ATI 1505
Pre: reference 30

What's the most economical way to get better sound? Replace the Kappas? or get a sub? If replacing the kappas, is it best for HT to have a matched system and go with definitive for the front channels as well? Any thoughts would be greatly appreciated.
I would replace one or the other, keep the brand you think are the best sound of the two. One thing is for sure, Kappas sound nothing like Definitive brand of speakers. Your system will sound much better if you stick with one brand all the way around. Your system looks like a great system that will only gain with better speaker match. I will guess that you mean B&K reference 30 as your pre?
I'm sorry- the preamp is B&K. So you think I should change the main speakers before bothering with a sub? Thanks for the input.

Use the same speakers all the way around. Avoid sideways center channel if possible. These are designed more for looks and marketing than for sound quality and the correct dispersion pattern (There are a few exceptions on the market). Also, avoid dipole/bipole surrounds, these were designed for diffused sound field recordings and not the pinpoint imaging capabilities of Dolby Digital and dts formats. Think of your system as a 5/6/7 channel stereo. Proper speaker placement and set-up of your pre-processor is the next step. Adding a sub should be on your list, especially if the fronts are not full range. Make sure the sub is placed up front. You may need to move it around to find the best location for your room and blending with the front channels.
Thanks! Appreciate all the input! Any suggestions for a sub? should that match the other speakers as well? or is it less important with a sub. Would buy used up to 800.

You can try DefTech 2006 TL for your L/R, list $1200 new. They are great for HT application and have integrated powered sub, BTW. I auditioned them with McIntosh amplification against Paradigm Ref. 100v2,which I ended up buying (primarily for music), I was quite impressed with DT's for HT.
The sub does not need to be the same make as the rest of your system. The crossover may not be an issue either because your pre-processor has an electronic one built-in. Run the sub crossover off or wide open then set the volume level with test tones and a meter. There are a number of good subs out there. REL seems to be a favorite for musicality. M&K and Velodyne is another good place to start. Try a search on best subs in the archives.
At the risk of opening a debate that will never be settled, I would not agree that you should "avoid dipoles/bipoles". True, the shift to DD5.1 convinced many people that direct radiators were a better bet than dipoles, but there is by no means a concensus on that. Specific brand, placement, room acoustics, and many other factors enter into the equation. YMMV. Listen to both.
good news (since I own dipoles :) But what about the bass? Better to go with sub or match buy matching front channels-- big difference noted b/t infinity and DT. What would be a better value?

I would match your speakers first. Move the Kappas to another room. Use B-system (multi-room) speaker outs on your receiver if it has that feature. If you have no use for them, sell them and add that to your theater budget.