I just dealt with the same issue and researched it to death. Most people will tell you to buy a new house before placing a speaker sideways but if you have to you can make the best of it. I would avoid those def tech 55's, I am not familiar with them but that top mounted woofer will be aiming right into the wall of your cabinet if it is sideways. You can look at KEF speakers, they have concentric drivers which are supposed to be ok for placing horizontal but are kind of expensive. I ended up going with the HSU hb-1, not as ideal for placing sideways but they do have a horn tweeter that can be rotated to direct the sound, and they have pretty good reviews. I am very pleased with mine, if you keep them all the way to the front of the shelf anduse bumpers to aim them at the listening position you should be ok.
I would reconsider keeping the center though, either get a matching center channel or lose it entirely. The center is supposed to match the left and right for seamless surround effects. It may sound funny hearing a helicopter moving from left to right with unmatched speakers using different tones. The HSU speakers are 499 for the 3 speaker package including the center.
You could also go with a shorter speaker like the cambridge s30 and just keep it vertical in that spot, they are 9 inches high. They are only 220/pair, and there is a huge thread here with some great feedback on how they sound.