How can I improve my HT audio?

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.

Peter

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.

yes, a sub will make or break any 5.1, 6.1 system