Interface upgrade or just converter

[VO-Guy]

Hey guys,

Just wanna say thanks to everyone here for your valuable input. Here's my question, I do VO work and here's my setup....

Mic's- U87, Sennheiser MKH-46
Pre's- Avalon M5
Comp- LA 4
Strip- Focusrite ISA 220
Interface- Presonus Firebox

I can't help but thinking I'd benefit from a better audio interface. BUT my question is, do I upgrade the interface with say a Motu or Fireface 400 OR keep the interface and just go with a great A/D-D/A box like Mytex, Apogee Minime or Lavry and go in via Spdif into the Firebox? Or doing VO work would I get just as good a sound from the Echo Audiofire series and save a few bucks. I'd love to get a Rosetta 200 with Firewire and solve all my problems but that's alittle out of range right now. Thanks for all your help!

Russ