Not surprised, that amp is pretty cheap.

Here a few amps to look at for 2 speakers:
1) CV B52 $110ish
2) NVX VADM4 $160sih
3) JL MX280/4 $250ish
4) Soundstream PN2.350D $90ish

 

For Kappas, Id go with B52

 

Shame, Amp sounds great except for the BS harness issue.

 

If you think that amp is great, you're going to be in shock at how well any of these other amps work for you. The PBR300 is a turd. There's just no other way to describe it.

 

I have that set up and it has proved well for me. I can hear at 80+ no problems

 

Are any of these amps plug play for a 2018 SGS?

 

No amp is plug and play as you have to take off the tank/seat just to run the amp wire harness. With that being said, you can go aftermarket amp and speakers without cutting any factory wiring and voiding the warranty. A bt355 line leveler will plug into the speaker outs off your boom HU, a set of rca lines from that into your amp and speaker wire from your amp to your speakers.

 

FYI Stinger 700x4 $175.68 on Amazon

 

Overkill for a set of Kappas.

 

I think I might have a B52. I'd have to look if your interested.