Whats diff between. Ht bypass vs using a passive connection-unity gain

High quality a/b source switcher?  Didnt know there were any.  Remote control? 
This would be awesome to find.

integrating a highend tube preamp into ht is a nightmare!

Maxwave, 
abit hard to follow but it appears it is risky if u screw up ?  Guess most have this serious problem.

HT pass through is a feature.

Unity gain is a state of 0 amplification/attenuation.

I cannot speak to wiring/specifics but I believe this:

A HT pass through will remove/bypass any amplification/attenuation from the signal path. Think of it this way: When you engage HT bypass, it is as if the preamplifier is completely removed from the signal path. You are sending the signal directly from your A/V processor/receiver to your two-channel amplifier.

When you disengage the HT option, you will revert to the last volume setting. Presumably, this will not be a setting that can cause damage to equipment or hearing.

If your preamplifier does not have a dedicated HT pass through function, it will depend on your preamplifier what happens next.

As far as I know, most preamplifiers are attenuators. Therefore, to achieve Unity Gain (a state of 0 amplification/attenuation), the "volume" must be turned all the way up.

Of course, this leaves open the potential for a situation where, when switching back to a different source, the volume will be at a very high level.

integrating a highend tube preamp into ht is a nightmare!

That depends on the preamp. You have to understand that the preamp is not designed to be part of the signal path.

Either the preamp has a HT feature, or it doesn't. If it doesn't, than your 
HT signal is just another input. Your only option is, when you choose that input and assuming your preamp has 0 amplification, to turn the preamp all the way up and forget about it. All control is done from your HT device.