Most likely no -- and I don't really see why Amazon would wanna add such capability even if they could.
What you *could* realistically try is use CEC functionality, if you just want some very basic things like turning on/off everything. But that's got nothing to do w/ the Amazon Fire TV cube...
That's really entirely up to you.
IF ARC is working fine enough, I'd probably just stick w/ that (for the sometimes modestly better audio... depending on the source). But if it's too flakey, then switch to optical.
_Man_
Well, the higher bitrate lossy tracks can still sound a bit better than the more basic ones that optical digital supports. Not sure if that's the diff you're actually hearing though...
_Man_
AFAIK, plain old ARC can be slightly-to-modestly better than optical... but the audio is still restricted to lossy surround -- it allows DD Plus and the like for higher bitrate lossy.
eARC would allow lossless surround, BUT if you're just using a streaming service, those are generally all using...