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

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.

@jumia,

What pre-amp are you using? Is it an active pre-amp or a passive pre-amp? Or does it have a combination of both active and passive output choices?

It is the schiit freya preamp which has passive output and a buffer output for tubes.

so that mixture of outputs can be risky.  Not sure what else passive would be used other than with ht avr, albeit not ideal  Not sure why schiit cant simply allow signal pass thru with out involving a passive output.