How good are modern multi hdmi super duper 50 channel receivers compare to vintage

class d tends to sound more dry and analytical. The better vintage gear will sound warm in comparison. There are of course exceptions. Class AB high current low feedback can sound really good regardless of price. 

The more complex the circuit pathway the worse the unit will sound. Class D or otherwise.

I'm not aware of any modern surround receiver that sounds like an 'audiophile' stereo unit. I'm not aware of any of them having a simple clean signal pathway.

On the older units, which come close to such an ideal...you might have to upgrade some of the smaller capacitors.

for example, on my computer monitor, right now, I'm using a RX-V1500 Yamaha, where the entire input section, the preamp section and the digital signal handling aspects of the front left-right section have been rebuilt with all premium non magnetic components, and coupling caps, with non magnetic poly bypass caps, and all the way into the main l-r power amp section..which as also been rebuilt and buffered, etc, capacitor-wise. I cannot abide mediocre gear.

HDMI has no reason to go thorough such a device, if one does not change sources constantly. Most people don't ...and end up with a whole bunch of complex switching --that they rarely use. I go HDMI direct and use quality older receivers that have no HDMI switching. Marantz SR-18U comes to mind. A beast of a power section. (the last older one I had my hands inside of was an SR-18u ..and the amp section is well built.)