The only way you can make a good stereo system into a good HT system is if you have the correct L&R speakers first and then add a matching center speaker . They should be speakers with a Directionality coefficient (Q) high enough to reproduce a signal with high intelligibility. Speakers that spread the sound all about the room don't work for HT. So if you think Line array and Horn based speakers don't make for good stereo, well its your loss. Because point source don't work for HT. Once you have your 3 front directional speakers then you have to decide what type of surround speakers you want. If you want the surround speakers just to recreate the ambience of space, for a concert recording or older movies, you can use point source speakers or bi directional speakers. But if you like todays current movies with sources of sound defined as they travel about the room then you need directional speakers. Line arrays or horns. Because the side and rear speakers are normally closer than the fronts the directionality of the fill speakers doesn't have to be as high, and because the bass is handled by LFE speakers they don't have to be as big either. Its a delicate balancing act. The more effects or fill speakers, the louder they sound the more directional or closer you have to sit to the front speakers. If your room were a perfect anechoic chamber with no reflections then the directionality of the speakers could be scaled way back, But the more reflective surfaces you have the more directional your speakers have to be. Also the furniture in the room should be as absorbent as the humans in the room. So the sound balance doesn't change with the room empty, full, or partly full. That means leather and plastic seating are out. Floors should be padded and carpeted, walls and ceiling absorbent. This absorbs stray sounds, decreases the need for speakers with very high Q. But also demands the system be able to put out more undistorted sound as the system doesn't have the room acoustics to help the system as a stereo system would desire
Stereo systems depend on the surfaces of the room to re-enforce the ambient sound on the recording to make a recording seem more life like. The problem today is most recordings are contrived in a studio using digital effects to simulate the atmosphere of where the performance is suppose to take. Knowing what the engineer intended can only be recreated by using highly directional speakers removing the sound of your room allowing the studio sound to shine thru. So where some folks think directional HT speakers don't make for great sound, actually those same speakers are the only way of you hearing what the recording engineer intended.
Now you know why LPs have made a resurgence. Engineers in the past wanted to capture the live sound of a performance in an acoustical space. it wasn't until Les Paul came along effects started being part of the recording process. The idea was to keep the sound real. Later on with the introduction of the Moog and other gimmicks during the disco age that pop music was transferred from on stage sound to studio sound. Studio sound was crude at first so speakers that spread the room around the room to add smoothing room acoustic were still the first choice by most folks. But as things have changed so must your system. How you choose to do that is your choice. Directional or non directional speakers for stereo. HT is easy. Directional speakers with the understanding the center channels is the most important. Not an after thought.
We have talked about the quality of the HT system, Now we have talk about quantity. point source speakers are usually confined to below 92 db sensitivity, with only the big Coaxial speakers approaching 100 db sensitivity. Home theatre systems should be able tp produce 110 db from the center speaker. which its quite easy for a Speaker system made by JBL, Altec, and Klipsch. 5 to 20 watts is all thats necessary. Point source speakers would need any where 75 to 400 watts. And if you plan on producing the peaks, you might need 1000 watts. Tannoys, Altecs, JBL, other directional speakers might require 100 watts or less less. Where line arrays fit in this situation. Well back in the 70's the only way to produce 115 db as Paul Klipsch recommended with less than 1% distortion was with a line array spreading the demands across many speakers working to gether. It wasn't until the very late 80's that low distortion was extended to the woofers using new woofer voice coil and magnet design, with multiple drivers. So for low distortion the line array was the solution. Today with JBL and others using the same woofer technology and extending the magnet design to the mids with modern rare earth diaphragms for their drivers lower distortion was added to horn speakers. Klipsch redesigned their speakers, too. Other manufacturers have incorporated the new technologies, too. So today we can have point ounce, Horn or line array speakers only requiring different amounts of power to reach similar levels with the same low distortion.
You can use any type of speaker with low distortion for stereo. But for HT you still need directional speakers unless you are willing to make your HT space an approximate anechoic chamber. Its just more economical to use a good Horn loaded system and then only the most recent ones to guarantee the lowest distortion, with the largest dynamic range with moderately powerful AV processor- amps.
The question is do you as the consumer need all this performance. The way the studios mix the DVD sound you have to turn up the volume a certain amount to get a comfortable speaking voice level. Once that that is established your system has to handle the built in dynamic range of the recording . Some units have a night switch to help control levels. I would like to see more programming choice allowing us to limit the dynamics of some movies with out destroying the intended artistry. The other choice is to use amps with built in limiting to control the dynamic range or add limiters to the system. The film industry and the HT designers of electronics, don't make it easy or cost effective.
Now as others have said as a HT processor the C-39 time has expired a long time ago. It has a lot of flexibility and if you want to add some speakers to add ambience to a room for Stereo recordings, it does that quite well. But for producing current movies, nope, no way, no how. We had issues with every C-39 we sold, MX 130, too. But they sounded good in the stereo mode.