SMSL 98E killed my setup?

So today I got the SMSL SA-98E amp from amazon after waiting for 50 days. I was pretty excited since I could finally test out my new passive speakers. My setup up to then was a scarlet solo and a jds atom amp to drive my HD600s. So I unplugged my headphones, connected the rca outputs from the atom into the rca inputs of the smsl, and hooked the speakers to the smsl. When I tried to play music, the speakers made a high pitched buzzing sound not too different from a sine wave, and there was no hearable trace of the music I was trying to play. I started troubleshooting, rechecking the speaker cables were connected properly etc. I noticed that the buzz became much quieter when I unplugged the rca inputs of the smsl, and upon reconnecting said inputs, the speakers made a very loud sound like when you touch a live guitar cable; I immediately turned the smsl off, but the atom amp and the scarlet solo didn't survive the ordeal. Their lights were off and they showed no sings of life. Since then, my atom amp has turned back on but it outputs an
obnoxious sound like the one that came out of my speakers, even without any input at all, and my scarlet solo hasn't turned back on.
Have I killed my scarlet solo and atom amp? If so, was it something I did wrong with the wiring or can plugging a defective amp kill other components through rca?
Any help appreciated

@roberjerman Could I plug a scarlet solo into the SMSL? Or would I need to use a rca/aux adapter from my phone/pc?

You can connect a preamp into the aux input (or any high-level inputs) of an integrated amp. Just keep the volume control low so you don’t overdrive the preamp section of the integrated amp.
This is correct.

From what I understand, the impedance mismatch made the signal backfire into the amp and scarlet, frying them both.
An impedance mismatch would only affect sonics, if it had any effect all. It would not cause damage.

I could accidentally press the GAIN button on the atom amp and kill everything, so I would rather just get a new amp and be safe.
The high gain mode of the Atom provides a gain of around 12 db, which is perfectly reasonable for a preamp. Using that setting won’t hurt anything as long as the volume control is not cranked up too high. The recommendation in the manual to set the gain to low when using the RCA outputs is mainly intended to optimize the part of the volume control’s range that is used in typical setups, and perhaps also to optimize noise performance and sonics. And in some systems the low gain setting will not even be suitable when the RCA outputs are used, since that setting provides no gain.

Question: When you plugged or unplugged the cables connecting the Atom to the SMSL, did you have the components turned off? If not that is what might have caused the damage.

-- Al

You can connect a line-level component into the SMSL safely by having the volume pot turned off.
Question: When you plugged or unplugged the cables connecting the Atom to the SMSL, did you have the components turned off? If not that is what might have caused the damage.
 The atom was turned on, and I think the smsl was also turned on. I also had the volume cranked pretty high on the atom, with GAIN turned on.
I'm still surprised that the atom and the scarlet died but the smsl is intact
Plugging and unplugging live components into each other can cause damaging transients! Don't do it! Turn everything off before plugging/unplugging cables!