analog in will NOT bypass pre-amp of 705. Only that it does not have any "digital decoding". Analog inputs may still be converted back to digital and processed for bass control, crossover etc. Unless you use a pure direct mode which doesn't apply any processing. But "pre" - that's volume control will still be there I assume.
I think you have several options
1. buy a pure stereo power amp - in lyrita pre there are multiple 2 channels inputs that you can switch. Connect pre outs of left/ right from your avr to valve pre. So when watching movies, or for any reason putting avr to duty, the stereo power amp will still power your fronts. There may be "too much gain" as onkyo pre will also provide a gain, and your valve amp will as well.
2. Get your avr modded by any onkyo service person - to give you a direct tap into power in, and preferably a "line-out" instead of pre-out. So that whenever you are ready to move to option 1, you get the best results.
3. buy a power amp and keep two sets of speaker cables. This requires you to switch cables manually (or use the switch created by venkat) but keeps your music and movie systems separate, with no mods needed.
4. buy pre/ power/ integrated/ speakers from lyrita (I am assuming you are going the lyrita way) and keep two complete separate systems for music and movies.
I am also in the same situation as you - I have a lyrita pre that I want to put in use. But exploring options. I think 4 is the best and I would go for it soon. However, I will experiment almost all options that lie in between.
I consider your need to use 705 amp section only & not preamp,
in that case using 7.1ana input,which bypasses preamp is best way.
If 705 has pure direct switch,it shld shut other vid processor sothat full power can be used by amp section.....