Almost reluctant to respond, but as I have the experience.
I had a 7.1 all tube, almost all OTL surround set up. The beastz driving the fronts, the Mini Beast driving the sides, the CFA driving the rears, and a Dynaco ST70 driving the center and ".1" bass channels. Four GGs driving each amp. An Oppo Blue Ray DVD 7.1 player as the source. The following are my impressions and things I believe anyone should consider.
First, a space suffcient to encompass everything and for the sound to disseminate. Mine was 4 meters by 9 meters. I had a dedicated 20 amp outlet which powered everything, and it was at its limits. I had to custom make a display case to place everything on, and with that much gear it could look ostentatious if not presented properly. The heat generated, even in the middle of the winter, meant opening a window or two. If I had it to do again the case would be enclosed with its own dedicated cooling source.
Front, side, center, .1 and rear speakers where carefully chosen and assembled or modified. Unless it is for just one listener or viewer in a focused seat, movie viewing or even 5.1 SACD listening, IMO should lean towards a wider/off axis speaker. These are usually of lower efficiency and a bit harder to drive by a low powered OTL than the high efficiency full rangers we often use with OTLs that have a pronounced beam and are therefore unsuitable for surround sound systems. It is entirely possible to put together two and even three way systems with 91 to 93 SPL drivers that cover the spectrum while throwing wide dispersion, and are still small enough to mount on walls for the sides and rears. Then, even the 4 watt CFA or Mini are powerful enough as side and rear power amps.
It takes a while, using 4 GGs, to balance the sound front to rear and side to side. For that reason it is also necessary IMO that each and every channel have its own dedicated attenuator. (One could eliminate the preamps and have the volume controls on the amps and take the DVD player directly into the amps. There are a number of small tranny-coupled two-tube power amps out there that could be fitted on a chassis that although larger than the standard, still small enough to fit in a number of rooms.)
Besides being very expensive as compared to a Bose “out of the Box” all in one 7.1 system for example, it is not really practical. More often than not I would just listen in stereo rather than fire everything up. There's the old adage that you only have two ears and so you shouldn't need more than two channels. Yes, IMO this works entirely well for listening to recorded music, which is usually, even live, presented in front of you.
However, movie watching and its subsequent surround soundtrack are a whole other world. And although impractical, the sound quality of the above system took movie viewing into not just another level, but into another realm. Much more three dimensional and immersive, as compared to an out of the box 7.1 all in one system, or even as compared to a THX digital extreme system in a commercial theatre. The only thing that could be improved upon is the replacement of the GGs with MPs. There is no comparison, OTL amplification does to movie soundtrack playback what it does to recorded music playback. And TS OTLs are by far the best.