Amp suggestions for Kinergetics SW200?

I have Spica TC-50s paired with stereo Kinergetics SW200 subwoofers (rated at 150W x 2).

I have been having some trouble with the amp that came with the Kinergetics and wanted to try something else to see if I can resolve some riddling at the lower frequencies (which apparently is a common problem with the amp).

Someone suggested the ATI 1502, but the full range amp seems overkill to just power the lower frequencies of a sub. I also don't want to spend ~$600 on an amp with only a possibility that it will solve the problem.

So... I don't mind buying used. If someone sees something in the classified that looks like a good deal that they can point me to that would be great. I know very little about amps!

If no one can find any good used amp deals, some suggestions on new amps would be great, too.

Thanks for your ideas!

You could get a used Bryston 3B in your budget range (or maybe even a 4B.) Yes, they're full range amps, but they come with a transferrable 20 yr warranty, are great performers in your application, and are extremely reliable.
Riddling? I'm not sure what you mean. Do you mean rattling? A different amp may not be of much help. This subwoofer has inherent limitations. Kinergetics made decent amps. The Kinergetic kit was designed to fill a particular need at a particular price point.
I believe that amp has a built in crossover. You might want to consider a Sumo Delilah crossover (stand alone) for around $200.00 used, (they've gone up), then you can just find a clean Adcom 555, etc., and tweak all you want.
the woofers have excursion limitations and are prone to bottoming out with the stock amplifier adding a better more powerful amplifier really won't help. Would suggest a Muse 18 subwoofer it's much better overall and blends better in the crossover region.