PARASOUND HC-3500, definitely! There's 2 for sale on this site right now...
If you can find one, an old Perreaux PMF-3150 will supply you with a very noticeable increase in bass output and control for not that much money. This is the bigger brother to the 2150 and is rated at 300 @ 8. In a direct comparison between one of my 3150's ( i have two and use them for multiple subs ) and one of my old 2150's, the 3150 was much more powerful and made the 2150 sound lean and anemic.
While the difference may be partially attributable to the increased power output, the mass majority of difference is probably related to the redesign of the power supply section. Peter did this when trying to further improve the design of the 2150 but was unable to retro-fit these changes into production for the 2150. As such, they ended up being incorporated into the 3150 series, which came out just a bit later ( along with the BIG boy 5150 ). For the record, the 3150 is of a high current design, biased quite high into a rich Class AB and also has very high rail voltages ( 90 volts as compared to most others at 45 - 70 volts ). It also runs pretty hot.
Most of the newer Perreaux designs of the same appr power rating as the 3150 are all pretty well known for having awesome bottom end. John, the owner of Chicago SpeakerWorks ( also called Van L SpeakerWorks ) recently auditioned the current 300 - 350 watter and stated that it had the tightest and most prodigious bass that he had ever experienced. It was also sucking the 20 amp circuit breakers dry : ) Art from Listener mag also commented on how warm and powerful a little two way three driver MTM array sounded when being driven by a newer Perreaux amp at an audio show.
The Parasound 3500 might also be a good candidate for the amount of money that you mention.
One thing to take into consideration is being able to match the gain characteristics between the two different amps. The Perreaux's take a little more drive than a lot of other amps, but you're probably already aware of that. Sean
I run a Rotel RB-991 for my pair of DIY NHT 1259's. I also have a Parasound HCA 1500A. Both are 200 watts x2 at 8 ohms and are plenty of juice. If you want more power, go with the Rotel 1090, or a similar Parasound. But I agree with Lthkeepr, don't spend more, you are wasting money. If you want to go even cheaper, Adcom GFA-555's can be bridged to put of 600 watts at 8 ohms. You can buy two of these for $700. Just some thoughts, good luck.