The Amp does a pretty good job with the phantom center channel, but a dedicated center channel speaker or driver from a sound bar will always sound better with dialog.

Another feature to consider on the Arc is the upward firing drivers and Dolby Atmos compatibility. Listening to music in Dolby Atmos is probably my favorite thing about the Arc. It has forever changed the way I listen to music.

If I were you, I would test out both options and see which one sounds the best. You can always return one of the items.

Thank you.

I would start by carrying over the AVR and wiring the speaker pair that you like to the two front channels of the AVR, flanking the TV. That set up in stereo mode will work well for just music as well. You can wire the centre channel of the AVR to the existing centre channel speaker, for use with TV in 3.0 mode. Only this will allow you enough control over dialogue levels so that you do not suffer the problem of having dialogue not well heard in effects heavy movies where the sound has to be turned down because the effects are too noisy. 

I believe that the presence of a centre speaker and what that does to dialogue is more valuable than surround speakers.

If the stereo speakers you like are bass capable, you will not much miss a Sub in this set up.

All of this is easy to test out because it does not involve any new purchases, except a source for streamed music, which could even just be an Echo Dot wired to the AVR.