Im trying to upgrade my setup and over the last few days I have done hours of research and everyone is suggesting different things

I currently have the m-audio firewire solo with the Audio-Technica at3035

I have under $1,000 to spend and wanted to spend it on a good preamp

Then I heard since I'm using the Firewire Solo, the quality will still be horrible

So people suggested the RME Babyface or the Duet2 to replace the Solo.

Then some newer suggestions were since Im using an iMac I could get the FocusRite ISA One Digital and since it is digital out, I could hook it up to my iMacs digital mini-toslink input

The ISA was my favorite choice and what I was going to get until someone said that I won't be able to monitor the sound unless I use the firewire solo

But on the site it says this Headphone output with volume controlISA One can feed either a sum of the two inputs to the headphones output, or an external stereo cue mix (such as a stereo mix feed from an interface) via two TRS Jack inputs on the rear of the unit.

So my question is.
Wil I get the best quality from using the ISA to my iMac and will I be able to monitor with the ISA without using the computers audio device?
Or should I avoid this and go for the RME or duet2?
Another thing to add... Im probably going to be getting the Shure SM7B pretty soon as well

This is the hardest purchase decision of my life so any advice is greatly appreciated!
Thanks