The 300x2 is the most entry level of entry level amps and you found it's achille's - heat. Stay far away from that PM100, not even remotely worth it. The RF TM400x4 is a good amp and a lot of guys are very happy with it in bridged mode. It's underpowered as a true 4 channel amp, but bridged it works well. Even the 2 channel version would work well with the 2 ohm Kappas if you're only running fairing speakers.
If you want to stay on the budget end of the spectrum the CV B54 or B52 is a good match for those kappas. If you're willing to invest in biketronics stuff problem solved. Lifetime warranty, unbeatable customer service, and a solid product. Only downside is no knobs for fine tuning, but they provide gobs of clean power that is plenty for most applications. I've ran their 2180, 4180, and have the new 4250 sitting on the shelf waiting to install.
Freedom isn't free, remember to thank a vet.
2018 CVO RG lava orange, 117 Stage 3, 14" F47 and a few chrome extras. Boom crap gone.
2016 RG Special Cosmic Blue, 120ST with Dragos 2:1 and TTS tuner, FBI drop seat kit and JNR air ride. Stock audio long gone.