I have had this amp for a week now. I am using this in a 2 channel music setup. This was bought as an interim amp until funds are available to upgrade. I own Audio-gd gear (Phoenix head amp and preamp, Ref-1 DAC). I had the FBI-500 but sold for money reasons. As compared to the FBI-500, the UPA-2 is definitely less refined across the board but it's not a fair comparo as the 500 is 5x the money.
What the UPA-2 offers is a quiet background, bass that is full but slightly loose- could stand to be a bit more refined. The mids and treble have a slight glassy character to them but are still revealing enough without being harsh or grainy. The 125 rated watts are real watts, not the Sony receiver 125 watts for sure- plenty of headroom for moderately efficient speakers.
If this sounds like I am overall unimpressed, not true. For $300, this is a very impressive amp and I can't imagine much better even used at that price point.