HT Bypass on preamp

The 2ch preamp in bypass mode is simply passing the signal from your processor. In this mode, the volume control on the preamp is disabled so that the A/V receiver volume control is used to drive your L/R speakers and amp. 

Some preamps just set the volume level to unity gain and pass the HT signal through the normal signal path. Other preamps direct the HT inputs directly to the outputs, bypassing the preamp circuitry. You need to look at each preamp to learn exactly what it does with the HT bypass input.

Setting unity gain is a simply way to do HT Bypass and is therefore very common. In that case the volume is control by the receiver but the direct connection to the power amp is from the preamp circuits. As soix says, you can create your own HT Bypass using any input and a known setting that is unity gain.

The Ayre K-5xe(MP) manual describes HT Bypass as setting the volume to unity gain and disabling the volume control. Not sure if that applies to all Ayre preamps, but it is quite possible that is a common design.

The Pass Labs XP-30 manual also describes HT Bypass as setting unity gain. Again, I am not sure if that applies to all Pass Labs products, but it is quite possible it does.

My preamp (Cary SLP-05) actually reroutes the signal through a hardwired connection directly from the HT input to the output, thereby bypassing the circuitry  It fact, you can do HT Bypass with the preamp off. When you power it down, it sets the relay to do the hardwired bypass. This can save having to have the preamp on when using the receiver.

Either method works and I am not sure there is really an advantage other than the hardwired bypass option allows you to have the preamp power off when using the home theater.

My Musical Fidelity A5 Int Amp also has HT Direct that completely bypasses the preamp section and applies the inputs from my AVR preouts directly to the amplification section.

There is one benefit to this type of 'true' bypass as opposed to simple unity gain thru the preamp section.

IF you have a sub that has both LFE inputs and line level L/R inputs, you can have a sub for both AVR (with the sub output from the AVR to the LFE sub input) AND 2-channel (with preouts from my A5 int amp) without the need for swapping cables or some sort of switch in front of the sub.

I have this setup in my hybrid HT and 2-channel system; works perfect.

I don't see your setup being any different with unity gain or "true" bypass. What you are describing is the advantage of having 2 outputs - one amplified and one not. This situation relates to integrated amps but would be pretty rare for a preamp, as most preamps do not have 2 outputs.  In any case, the path the signal takes to the output should not matter. 

Plus, in the case of the 2 channel source, it seems like you are sending the low frequency both to your sub and to mains.