I own the SL12si's. Dragged them out of storage while my DIY speaker project takes form. I've always loved Celestion monitors. Mine are over 10 yrs old. Are yours the ones with the aluminum honeycomb enclosure? If they are then definitely keep them, IMHO. If you go out and buy some a new set marketed for HT, I can almost guarantee they won't stand up to the 700's. Look, I don't do HT. Speaker are speakers. Anyone can split hairs and say these new HT speaker are design for the clearest repro. of voice, etc, which is why so many are D'appolito or MTM arrangments. The most important difference in HT speakers is the bass. Movie soundtracks are recorded with an inordinant amount of deep bass, like the sound of the T-REX footsteps in Jurassic Park.
OK, back to the subject. Unless you want better speaker for music, keep the Celestions. For a center channel, you could pick up another pair used and use one for the center. That's actually a pretty inexpensive solution. You could use a 12, an SL12 right?. but I wouldn't. it's a different speaker, different crossover. If you ever need to replace a tweeter, etc. That can be done through the mail, buy the part and install yourself. I recently did that with mine. If you have any more questions I'd be happy to answer them. I have enough technical speaker knowledge to answer most if not all of your questions without any vested interest.
OK, back to the subject. Unless you want better speaker for music, keep the Celestions. For a center channel, you could pick up another pair used and use one for the center. That's actually a pretty inexpensive solution. You could use a 12, an SL12 right?. but I wouldn't. it's a different speaker, different crossover. If you ever need to replace a tweeter, etc. That can be done through the mail, buy the part and install yourself. I recently did that with mine. If you have any more questions I'd be happy to answer them. I have enough technical speaker knowledge to answer most if not all of your questions without any vested interest.