Best Amp For Infinity Kappa 8.1 Speakers

High current is what you need. An Odyssey Stratos Dual Monoblock is well within your price range and will drive the Kappa 8.1 with ease.

I have the old Kappa 8 in a back up system. I've had them since they were new. I agree with Wccoe. The Odyssey works very well. I have Odyssey mono's in my main system. The PS audio HCA works very well too. The original speakers I think were designed around high power tubes like Audio Research. I have tried an number of amps and feel the way the guy with the McIntosh thinks. A substantial amount of clean power and they are great. The Macs may be good too. Old Adcom 555 will play but you'll get bad high end harshness.

Kappa 8s? I have them and also had them when they first came out. I agree with the high current statements. I powered mine with a Threshold T 200 on the highs and a Parasound 2200II on the bass. The sound was as good as it gets with the Kappa 8 or Kappa 9! I then used the Threshold to power them full range, I was not that surprised to find the Threshold play clear and with power to spare. The T 200 is rated class A at 100 watts into 8 ohms, but will double power at 4 ohms; the Kappa 8 will drop down to a bit less than 2 ohms in the low frequency, here, the Threshold puts out 400 watts a side! So, think about this double up as impedance drops in half. It is the gain in power between 4 and 2 ohms that is so critical for the Kappas, The Parasound is a good amp, but puts out about the same 350 watts into 2 ohms, as it does into four. Forget the older McIntoch amps on these speakers, or tubes, as they have taps the adjust the voltage and permit no more power at 2 ohms as they do at 8 ohms. The Kappa 8.1 is really not in the same class, but not so current hungry. Any big relitively high current amp will work on them. try the Rotel 1090, you can grab one on eBay for about $1500.
Charnet

Would the Parasound HCA-3500 power amp work well with the Infinity Kappa 8.1's?

The Parasound might be a bit harsh, espescially with Kappas. Adcom's GFA555 would be a better match. Don't get too concerned about the number of watts. I only have 100 watts powering my Kappa 8's but it takes 3 boxes weighing in at nearly 150 lbs.

The Aragon 8008 is a good hefty choice for solid state at a decent price. A strong tube amp has less of an iron-fisted grip on the bass, which the Kappas respond well to, but a good one will be expensive. These are very demanding speakers but can deliver awesome results with the right electronics.

The Parasound would be a decent choice for if you want to vertically biamp. You can use different brands of amps if you use a good quality attenuator on the louder. Pots sounded bloated.