I've been thinking some more on the subject of my original post but with a little twist: What would be a good combination of old & new that still doesn't lead you to buying a high price high end vintage receiver?
The core piece of vintage gear I would suggest is actually I think the 1st quad unit Sansui commercially produce, The QS-1:
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I have owned one of these & I can tell it's a real beauty in person. It, like two other units, is labeled a
Quadphonic Synthesizer. The other two units are the QS-100 & QS-500. They both have meters & rear ch power amps that aren't very good where the QS-1 does not. But the QS-1 is the looker. I suggest this because it is far less complex than any variomatrix unit ( the internal decoding is just passive matrix) easy to reach pots & switches for cleaning, sports my favorite balance control layout & is so easy to fix or do some circuit upgrades if you want. It has the usual 4 ch discrete input where, of course, the Surround Master would plug in. It might seem limiting with only 1, 4 ch discrete input but actually you would have main decoding modes, QS/SQ & the internal decoding of the Sansui. It produces very pleasant but not bombastic surround sound from stereo or QS. Because they are not the high sep Variomatrix type the cost is very reasonable. I've never seen one on Ebay for over $200 & I've seen buy it now as low as $125, and that's for units in VG condition.
Next you would want to match it up with a pair of minimalist power amps. I have a pair of AudioSource Amp One amps that have worked perfect for many years, very clean sounding. Those are long discontinued so I checked the AudioSource site today to find the two reaming similar amps in their product line up seem to be discontinued or simply out of stock at Crutchfield, B&H, etc.The do have a couple of nice choices in digital amps such as the AD1002 at 50 WPC bith channels driven for $179. You can search & shop to your hearts content elsewhere for there are many choices such as at Parts Express to find just the right amp for your needs & budget.
No tone controls, no remote control. But for about the same or less than what you would spend for an all in one quad receiver you have something fun & cool, great decoding with the SM & nice high quality reliable modern amps.