And you might just want to keep the mixer because these day's you get zero money for used gear , I never sell stuff that is still working. I just stuff it in closets or move it to a shelf where it can look cool. If you wait long enough they become collectors items. I like having a small mixer just to use for monitoring as I track. Most interfaces are very lacking in proper monitoring control.

I own RME stuff too, I don't use it anymore so I wouldn't go or recommend RME babyface. Any RME converter pack deal, old fireface or new RME UFX all sounds harsh digital sound.
 
* Note RME PCI Xpress card MADI ADAT! YES!!! those are great cards and products, very great drivers and good thing you don't need to use RME converters. YES buy and use SSL converters instead or other high end stuff!  Forget if you are on a budget below $4000!

RME= Great drivers, but bad sterile converters.  


You get much more bang for the buck buying EMU 1616M PCIe. Lower cost, high end quality sound, more channels then babyface and the new drivers are very stable. Converters are and sounds like Lynx high-end gear. Check it out!

http://www.creative.com/emu/products/product.aspx?category=505&pid=19007




About Focusrite ISA One. I own other focusrite products too. ISA sounds good, okay, but there are far much better stuff, DI and other gear for less that money or some or little bit more money that has actually more value per dollar. I would go for Gold Age Project.  NEVE replica, sound actually better than NEVE. It's just my opinion..

Cactus Music


And you might just want to keep the mixer because these day's you get zero money for used gear , I never sell stuff that is still working. I just stuff it in closets or move it to a shelf where it can look cool. If you wait long enough they become collectors items. I like having a small mixer just to use for monitoring as I track. Most interfaces are very lacking in proper monitoring control.

Keep the MACKIE and use it as patchbay are what I would do...

The only reason i still use the Mackie 1604 is for monitoring. I use 4 inputs for the tyros and Motif,  One input for my Joe Meek pre, two inputs from my wife's notebook setup outputs from the UA25EX  The channels are assigned to a pair of Stereo bus outputs, then sent to the Firebox (and/or her UA25EX to the notebook). 

Two reasons I still use the mixer, 

1. To monitor vocals with efx through a Yamaha Rev 4 without printing them.
2.  To have a normalled set up so as  not have to constantly patch synths, and pre amps in and out.  

I can do this with a much smaller mixer, but we had the Mackie 1604  24/4 and a 1402VLZ and decided to keep the 16 because of the 4 available sub busses.  

My wife has a notebook set up and a separate UA25ex interface patched into the Mackie for monitoring and Sub busses 1/2 feed her interface. So we both can use the same set up wihon our respective platforms without constantly repatching

I would consider a 4 input box IF it had reverb w/ monitoring only built in such as   the AudioBox 22VSL  .