I believe NAD has an amp and pre-amp combination at or near your stated price range (Stereo). I'm assuming stereo is your goal because reasonable vinyl sound appears to be the goal. If I recall, The Absolute Sound gave a thumbs up to NAD's moving magnet section of their budget stereo pre-amp. Think it was the c-262 or something like that. The power of their amp is not great, however, your Infinity speakers are efficient enough to not cause problems with moderate power.
If you give more specifics such as room size, type of music listened to, percent of home theatre applications, and features desired I'm sure some of the advanced audio enthusiasts wil give some good suggestions.
If you consider the intergrated amplifier option, many units from solid mid-fi companies would fall to your price range (NAD, Paraound, Marantz, etc...)
Good hunting