With the ProAc 2.5's an excellent choice is the Bryston 4B ST. I ran that amplifier with ProAc 3.8's and really was able to enjoy the music. The amplifier was able to control the woofers so that the bass was plentiful and tuneful. The combination also worked because the ProAc's, IMHO, lacked in dynamics and the Bryston, IMHO, was a bit forward in its presentation. Together their individual strengths made up for their individual weaknesses.
I am sure that their are many good choices that you could make. The beauty of the Bryston 4B ST is that you could find one used for around $1500 and spend the other $2000 on source upgrades that could make a greater difference. The warranty period ensures resale value. If you choose to spend more my suggestion would be to find a used Mark Levinson or Balanced Audio Technology amplifier. Good Luck
I am sure that their are many good choices that you could make. The beauty of the Bryston 4B ST is that you could find one used for around $1500 and spend the other $2000 on source upgrades that could make a greater difference. The warranty period ensures resale value. If you choose to spend more my suggestion would be to find a used Mark Levinson or Balanced Audio Technology amplifier. Good Luck