Luckily, my local Audio T (shout out to Andy if you are reading this) arranged a home demo for me of both amps. It seems very few people on the internet has down a side by side comparison. I am not a reviewer, not the most experienced audiophile but I thought I would share my initial thoughts. I have another day of listening yet so may update in the fullness of time.
The Set Up
All tests were done using my Planar 3, going into B&W 607 (first edition). Speakers are 1.5m apart, focused on the listening position which is 1.5m from each speaker. The room is a small room, and speakers are about 30cm from the back wall. There is no sound treatment on the walls. Volume knobs between a quarter turn to a half turn at all times as this went from comfortable to loud for me.
First impressions are that they are both great amps that pair well with my set up. The sound is clear and lively and definitely a step up from the Pioneer A10 I am currently using. The Brio definitely sounds bigger, with a slight more "umph" than the IO at the same volume. The character is good, and drums sound big and in your face. The IO has plenty more high end though, and to my ear, sounds a bit clearer. I found that I could take the bungs out my speakers with the IO, whereas they definitely needed to stay in for the Brio.
The test tracks:
Breathe - Pink Floyd
Warm and large on the Brio, but not as clearly as defined as the IO. Both sounded clearer with more personality than my current set up. The first track I demoed. Brio felt like it had more emotion to it for me (if that makes sense).
Sigh no more - Mumford and sons
Started with the Brio and it sounded big, but a little muffled. It was like the mid range and top end was getting lost. Turning it to the IO and the top end suddenly came back and you could hear all the vocals clearer.
Reprise from Spirited Away - Joe Hisaishi
Big song, and sounded lush on the Brio. Warm, big and engulfing. Switching to the IO it was noticeable that some of the power (although the same volume) was missing. There wasn't as much kick to it - however the trade off was that the IO was clearer and gave a better picture.
Helicopter - Bloc Party
This and the next song is where the Brio shone the most. Oh man the drums have never sounded so good. Punching me right in the face, matched with the energy of the song. Absolutely brilliant. Familiar story for the IO, more top end gave a bit more clarity, but lacked the energy of the Brio. This was the first song that made me think of taking out the bungs on my speakers and it did help a bit, but not a match to the Brio.
Millstone - Brand New
Again, big song and the Brio delivers. Massive soundstage and feels like you are in the room with them, with all the emotion that brings. The IO handles it well, just overshadowed by that power. The better top end makes it clearer to my ears again.
Harder, better, faster, stronger - Daft Punk
After the above experience I was expecting the Brio to win this one hands down, but surprisingly it was the IO that I thought sounded the better here. It picked out the character of the song better, and at no point did I feel ti was lacking.
Wouldn't it be nice - Beach Boys
The IO shows its talent in this one, picking out the instruments better and producing them clearer - again I was expecting the Brio to do better here. The Brio was good, but sounded muddy compared to the IO. Decided to listen to the rest of the album through the IO.
Final thoughts
These are two great amps, but with different use cases to my ears. The Brio has a big warm sound - it puts you in the music and makes it feel live. The IO gives the detail and presents it almost forensically but with a bit more of a human touch than that sounds. If I was going to be playing rock music, or big songs all the time, and at a high volume than I'd be going Brio all the way. However, this is going to be an everyday listener, and with some subtler songs at times. So for me, at the moment it is the IO.
I hope this is interesting/helpful for you. My recommendation would be to go listen to the amps, consider what you will be listening to and how, then make your decision. There isn't a bad amp between these two, just the one for you and the one not for you.