I agree with Jsd, more power. I also had a set of 90's and they were quite power hungry. I was driving them with a Threshold S350e, 150 wpc and while it did drive them ok I always felt the sound was kind of lightweight and not very punchy unless pushed really hard. The store I got them from would always demo them with a 200 watt or greater amp and it did make quite a difference in the punch and bass slam department. Also with poorer quality amps the 90's can sound very bright and fatiguing. Before I got the Threshold I used a Marantz 1/4 class A integrated amp, 105 wpc and while it was a moderately good amp the sound was a little on the bright side. The sound was a lot smoother and detailed with the Threshold but they could have easily used twice that amount of power to really make them sing. Do yourself a favor and buy a good quality solid state amp of at least 250 wpc and you won't be sorry. As to which amp to buy, any higher quality amp such as the Krell mentioned above or any thing else in comparable quality should be fine as long as it is a high current design with at least 250 wpc. There are a lot of older high quality amps for sale here for a reasonable price that should fit the bill. Hope this helps.