Buying a used amp ATI 1506 or Linn Chakra c5100 or RBH SA-200

ATI makes great amps.  They are an OEM builder for many companies.    When I think RBH i think speakers ...not a bad thing as they make good speakers.  Amp is made in China though .....   

My vote would be for ATI .      Made in USA. Excellent support and quality for the $$$

If you can swing it, I would go for the ATI AT1506.  It's not actually a Class A amp.  It's a Class AB amp.  You want as much current capability and resolution for home theater, even if you are not going to use the power.  A bigger power supply generally results in a fuller smoother sound (even at low volume levels).

The Lynn is a real lightweight amp at only 56 watts per channel.  I suppose if you want a low profile amp, you can get it.  But I would rather choose the ATI.

As far as the RBH sub amps, you would need to also find a passive sub-woofer (essentially a subwoofer without a built-in amp).  This is not something that is readily available by many manufacturers and may not be as good as other sub-woofers with built-in amps.  I don't think this is going to result in something better than your current Paradigm sub-woofer.  Plus, these are rated at 200 watts into 4 ohms, which really means it is ONLY a 100 watt amp (into 8 ohms normal specification).

If you want a better subwoofer, start looking at stuff like Rythmik Audio, or even SVS if you have to go low budget.