I used the Bryston 3b-st with the NHT 2.5 and was very pleased. The amp provided just the right amount of power and finesse for the speakers.
I then had a NHT 2.9 powered with the 3b-st and the 4b-st (horizontally biamped). The 3b-st was not enough power for the 2.9 alone and my biamped config was overkill. A single 4b-st would have sufficed for the 2.9.
So I would say that your 4b is more than enough power for the 2.5. I wouldn't spend more money on a real fancy (expensive/refined) amp for those speakers. Instead buy some good speaker wire. I switched from some Monstor speaker cable to AudioQuest Caldera for my 2.9 and the improvements (especially the bass) was surprising.
I then had a NHT 2.9 powered with the 3b-st and the 4b-st (horizontally biamped). The 3b-st was not enough power for the 2.9 alone and my biamped config was overkill. A single 4b-st would have sufficed for the 2.9.
So I would say that your 4b is more than enough power for the 2.5. I wouldn't spend more money on a real fancy (expensive/refined) amp for those speakers. Instead buy some good speaker wire. I switched from some Monstor speaker cable to AudioQuest Caldera for my 2.9 and the improvements (especially the bass) was surprising.