Yes, you can use the Amp. It creates a “phantom” centre channel. 

Sadly the Amp doesn’t do Atmos.

Several folks have gone a different route, keeping the Arc for TV and adding stereo speakers to use as dedicated L/R music channels. Fives are great for this but Era 300s, 100s, Ones all will work too but you’d probably want to add a Sub or mini-Sub to the smaller speakers.

Edit, just a thought repurpose your 100s as front music channels, add 300s as surrounds and if needed add a Sub. the 300s give Atmos capability to the surround channels.

I had a similar experience (ARC + Mini Sub + existing rears with Amp).

Tried the Era 100’s as two fronts, but when compared to my existing standing speaker (Monitor Audio R6), they (of course) did not stand the comparison (to be fair, when playing loud, in low volumes they are great).

So I added a second Amp, to have everything in the same ecosystem. And to my ears, the Amp with my existing front speakers sounds great. Really great.

I’m sure the 300’s sound great as surrounds, but they size/design of them means they won’t pass the Wife Approval Test.

I have Play 1 surrounds for exactly that reason. They are also poorly positioned, but sound better there than in a box in the garage.