Preamplifier sound Quality impact in home theatre bypass

It depends on the tube pre and the path the signal is taking for HT Bypass. 

In many of the designs I've seen, the HT Bypass is the equivalent of putting an RCA female to female adapter between two RCA pairs. Wouldn't bother me unless you noticed a change running with and without. 

McIntosh Integrated Amps and Preamps have a really slick pass through.  I set my Dad's MA8950 with an Anthem MRX 540 and it is seamless integration 

The MRX's trigger out goes to Mac's control trigger input and when you turn the MRX on it puts the Mac in Pass-through.  It's just using the MA8950 amps,  with the MRX doing volume and powering the centers and rears. 

I have a Rogue preamp I use with HT bypass. The AVR plugs into specific HT bypass RCA ports, which are only activated with the Rogue OFF. Therefore no effect on tubes, and unless the HT relay switch is defective, no impact on sound. A simple pass through. 

I’m going to have to agree with carlsbad2 on this one. I have a Parasound pre amp with theater bi-pass and never looked at it the same way as my 2 channel audio system.  Only a bi-pass for convenience…Everyone’s ears hear what they hear 

I've never had an issue.  Usually it's a straight wire bypass.