A few things may be going on.

a) Your Yamaha DAC may be better even though it's older

b) Your Yamaha receiver may be digitizing your analog signal, reprocessing it and converting it back to analog. By removing the AD process, you've improved the sound.

The question that you are looking for the answer to is this.
The D/A converter in your CD player has the capability to process 24 bit data at a 96KHz rate. The standard "Red Book" CD's that you play are 16 bit, 44KHz data. Does your CD player perform an upsample conversion from 16/44 to 24/96 or just pass what ever data source on as is? This info is often difficult to obtain. I have often asked for this info from manufacturers and from the response I have received still had no useful data.

The specs on many pieces of equipment (such as DACs) are now all good enough that we shouldn't hear any difference. Yet there are audible differences.

I believe there are a couple of things going on. Manufacturers list the specs that look the best for their product - not that they are lying but they are choosing. Specs also do not tell the whole story; for example, how the DAC chip is integrated into the rest of the circuit makes a big difference.

It's good that you experiemented! You found the best sound with your existing equipment.