Well the absolute BEST center speaker you could use with the HKB6's is actually another HKB6. A lot of people think that the center speaker has to be a MTM but thats not true. The reason most centers are MTM's is that the off axis horizontal dispersion is the best with a MTM on its side. Another HKB6 as a center may not have the same horizontal dispersion but it'll be a much better timbre match to your current mains.
Now as for DIY yes you can build one but there are a few things to consider. First it wont be an optimized crossover unless you have all the needed test equipment to optimize them (which isnt cheap). Its also a lot trickier to build main speakers than subwoofers. But it can be done! I've made a few unoptimized speakers so far and i've very pleased with the results. The biggest problem is that they will not be timbre matched to your HKB6's unless you use the same drivers. So it all comes down to how improtant the timbre matching is to you.
Heres a brief overview of how I've built my speakers. First I had to decide what I needed the speakers to do. The two projects I've done have totally different needs. The line arrays main driving factor was the cost since I was buying so many drivers. Thats really the only consideration I put into the driver selection. But with my DIY computer speakers I had a more to think about. I knew I wanted a very small package so that limited the woofers to no larger than 5". There arnt too many under 5" drivers out there that can really reproduce much bass with any power so I thought about ported them but that made the enclosures to large so i went sealed.
With your center speaker you'll want to find out what size enclosure will work in your room. Then set the budget for the drivers and see what is a available that will work in your sized enclosure. There are a lot of really nice midbasses that are around $20 or $30 and the same can be said for tweeters. You could think about doing something with the new dayton reference line midbasses. I've heard nothing but great reports about them and they arnt too expensive. As far as tweeters go theres a lot of different ones avaible. I would look at the FR graph of the midbass and try to find a tweeter that is around the same efficiency and will play low enough to match the midbass. If the tweeter is higher efficiency (which is usualyl the case) you'll have to add a L-pad to the tweeter to bring the sensetivity down.
Once you have the drivers and enclosure selected you can start playing with the crossover. I've used this online crossover calculator before and I've been pretty pleased with the results.
http://www.mhsoft.nl/spk_calc.asp
It is pretty straight forward and easy to use. But keep in mind these are not optimized. I dont know too much about making optimized X-overs but I know it takes a lot of really expensive test equipment. That said I think you can use unoptimized X-overs with very good results. Now as to the best crossover point that can be a tricky subject. It all comes down to how well your midbass can reproduce midrange freq's. It's nice to cross the tweeter over higher beacause then you dont have to worry as much about the corssover point being smack dap in the midrange which can sound bad. But with most tweeters you'll be crossing them over between 2,000 and 4,000 hz. I tend to prefer the higher crossover points. My AV 1's are crossed over at 3,500 , My line arrays are a little over 4,000 and my computer speakers are 2,300. The computer speakers sound a bit thinner and I think its cause of the low crossover point (which I am seriously thinking of moving higher).
So there is a TON of stuff to plan out with main speakers. you would get the best match to your mains and have the cleanest sound from left to right with another HKB6 as a center. But it is fun to toy around with your own design. If you do want to go DIY i would suggest you start out with some of the buyout Vifa drivers like the ones I used in my computer speakers. They are very cheap (under $10 a driver) and they are pretty good quailty and sound nice.
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=299-432
So you can get your feet wet and toy around with differnt X-over points and see how well they sound. Then if you want to take on a bigger project move to the nicer drivers etc.
Ryan
