Not liking the new Mac Mini 2018 sound

I recently got the new 2018 Mac Mini to replace an old Macbook Pro and am considering returning it. I'm wondering if anyone else has tried it in their system. Maybe I got a lemon and I need to try a new unit?

I'm using the i5 / 8GB / 256GB, 2018 Mac Mini to stream iTunes and Spotify (via airplay - AEX series g - Toslink or direct connect via USB) to my Peachtree that's feeding a vintage McIntosh system.

When I A/B it against my 2017 Macbook Pro, I lose a lot of detail and staging. The lows from the Mac Mini is sloppy and rounded with the mid's sounding thin.

I also A/B'd it using headphones and IEM's via audio jack and get same results. In fact, the Mac Mini sounds worse than my iPhone XS - how's this possible?

Would be interested in anyone else's thoughts who may have similar set up. I love the Mac Mini form factor and bought it with the idea that I would add RAM and external storage as needed. But the lacking sound output quality (maybe its the T2 chip that's causing the issue?) is a deal breaker.

I am surprised you hear a difference between the Mini and MacBook Pro. When you hear the differences, are you streaming wirelessly to an Airport Express or connecting via USB. 

Are all your iTunes settings the same, equalizer off and no volume adjustment? These can affect the sound quality.
Good question - I was careful in setting up the A/B tests. All same settings on both iTunes and Spotify, same volume, same power supply, etc.

I tested using both direct USB and wifi optical when using my dac. I then also tested using headphones connected to Macbook and Mini directly via 3.5mm jack. I chose two small pieces of music that I've been using for many years. The difference was noticeable.

I'm planning to take my IEM's to an Apple store to see if I can test using display models.   

I was considering the new Mini to replace an older MBP (circa 2011, 16gb RAM, 512 SSD) that would no longer support the new iTunes, and my iTunes library had already been updated on a newer Retina MBP (thanks for that Apple!!).  It seemed expensive after adding in the RAM and larger SSD.

Instead I bought a Roon Nucleus and am extremely happy.  I plugged my iTunes library into it with an external SSD.  Sound quality is better.  Mostly the noise level has dropped, so that I can hear more low level detail.