Do I even need a dedicated stereo preamp?

I don't see the benefit of adding a pre-amp to a multichannel receiver like the MRX 540.  At its price point being a receiver with all the other features it has I suspect the amplifier is not likely a strong point.  Adding a pre-amp will change the sound and you may like it but then your pre-amp will be in another class from your receiver amp.  You will be in the same situation asking if a separate amp will sound better but then the answer will be yes, definitely.

Save your pennies and get a good integrated or better separate amp and pre-amp. 

DAC’s with builtin volume controls are not all the same.  A digital volume control, as has been said, is to be avoided.  Many higher end DAC’s use standard analog volume controls just as a standalone preamp. For example I run my MSB Cascade DAC directly into my Amplifiers with great result. I also run my phono pre into the analog inputs of my DAC. Another standalone preamp would simply add color the sound.

I use a Bryston sp1.7 strictly in analogue mode which uses the Bryston bp25 as it’s stereo pre. One of these Bryston models could be used by you for both purposes and can be found used. I think it sounds great. 
 

You're using a Buckeye Purifi already, one of the best amps in the market, and easily one of the best values on the market. If someone is telling you that something will sound better than that Buckeye Purifi, they're lying, or don't know what they're talking about. Ignore the snake oil investors.