The TL:DR I like it and don't have regrets upgrading. Would recommend to new buyers and people considering upgrading.
I'm not an audio expert, so please bear with me with the terms and a lack of proper vocabulary. Having said that, there is an improvement. I was switching between the original and the plus while connected to the usb on my computer and playing music on Tidal. The first thing I noticed was that the plus was already cleaner and more detailed than the original, this is also taking into effect that the original had the a v5i op amp installed. I was achieving the same sound volume at the 12 o clock position on the plus that I had at the 1 or 2 o clock position on the original while using my 6xx headphones on low gain. I have the volume at the 11 o clock position on the highest gain.
Having switched over the v5i op amp into the Plus, the sound is even cleaner and more detailed. Tiny little nuances are more readily picked up. There is less noise as compared to the original as well from what I could tell. The original had an issue for me with the v5i where if I put it to the highest gain and turned the volume up, it would cut off and go into safety mode until it turned it off and on again. On the Plus, I had full range of the gains at all volumes. Vocals and treble seem brighter, but not harsh. Listening to Sinnerman by Nina Simone from the Pastel Blue album is amazing. There is a extra detail that I didn't notice before on the clapping portion. You can even hear the puffs on the microphone when she say "Prayin'" and "power". Very cool to get such little details like that.
AUX in is extremely clean and even less noise than the original one. Hooked it up to my Fluance rt80 record player and there was little to no hiss that I could tell. Very nice and marked improvement.
The bluetooth adapter works for both the original and the Plus, but it feels more secure in the plus than the original, or at least mind does. Works with the coaxial port and uses the headphone jack to secure it to the back. You have to charge it separately.
Don't get me wrong, the original was pretty good, but I feel that there are enough improvements to consider getting this one. If you're buying it on this site, I think that the difference in price would be worth it. If you already have one, that's your call. I would say do it and sell the original one to someone to cover some of that cost.
On a further note, some of the things I don't like. The input is a button on the side now instead of the switch up front. I liked the switch. Instead the switch is a thing called boost, which after switching it back and forth on USB mode, I couldn't tell a damn thing. The input button replaced the display button on the original, which when help down would turn off the display. I never had a need to turn off the display, so the loss of that means nothing to me.
The USB c charging and link ports on the back are simple and clean. Blue link light turns on during USB mode and red blinking light for charging when its plugged in. Iphone users will still need to use the USB camera adapter, but you will want to consider getting a short throw usb a to usb c cable, or else you'll have to use the long ass cable that comes with it.
Android users have the choice between a short throw micro usb to usb c cable or a short throw usb c to usb c cable. You have options there.
Hope that a random stranger's thoughts on the internet help you decide either way on ordering.