Using a HT system with a tube preamp

Will make a difference but not much. Problem is the vast majority of improvement is from getting out of multi-channel into stereo. Especially if its an AVR. You'd be surprised how much better movies are with really good stereo. https://systems.audiogon.com/systems/8367 Multi-channel can't touch this.

I hear ya. When I switch to eliminate the center spkr, it is hardly missed.

so using a stereo preamp vs a multichannel avr may be better.
but what about using an analogue preamp ?
Maybe a digital stereo preamp using hdmi connects may provide SQ that is very comparable to an analogue preamp which I guess can’t be used for HT use.  
The world needs to deal with this very harsh conflict between stereo and HT.  It is a real pain in the ass to piece together all the equip.  

There are plenty of excellent solid state stereo preamps that would be a huge upgrade over an AVR or prepro.  Also, there are a few tube pres that don’t have tubes on when using bypass — Rogue I think is one and maybe Backert or VAC?  Either way a good stereo pre will improve your 2-channel performance significantly. 

The world needs to deal with this very harsh conflict between stereo and HT.  It is a real pain in the ass to piece together all the equip

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Tell me about it. I must have wasted a good 2 years banging my head against this problem. Until it hit me, HT is pure gimmick. Its not really made for home theater, its made for selling movie tickets. In movie theaters almost everyone is sitting way off center so the only way dialog can sound like its on screen is to have a speaker behind the screen. At home it usually just a few and if your stereo images like it should then what do you need a center channel for? You don’t.
Surround is even worse. Pure gimmick. Once you realize this its pretty easy to just drop the whole thing. Especially if you ever bother to compare sound quality between stereo and HT components. There is no comparison.

So after a couple years I ditched the whole thing and never was happier. Can’t say it saves money, probably spend the same amount, but it sure does get a lot better quality for the same money.