the best computer interface less than $400

[The Cancers]

Well, I'm not entirely certain why you bought the RME.
Seems like the sort of thing you use to upgrade a fairly complete setup.
That being said, I don't think you need an interface with any balanced I/O.
See, the RME is gonna be doing a lot of the work that most interfaces already do, albeit better (I guess. I've never used it or heard it A/B'd with any of the all in one 8-channel interfaces). You don't want to send analog signals into whatever interface you get, because what you paid for with the RME is the conversion of those to and from digital signals.
You need an interface that will go between your computer and your RME.
So you need, basically, a box that has as ADAT inputs and a firewire or usb (or PCI, though I don't know if that's doable with a laptop) connection to the computer.
Since you already have a mixer with preamps, you just need to connect the direct outs from your mixer channels to the analog inputs on your RME, then the ADAT out from there to your interface.
I can attest to the quality of Focusrite products, though not this particular one:
http://www.zzounds.com/item--FOCSAFPRO24
The key here is that little "optical in" on the back. You don't want to send it any analog signals, because it will then do the converting of analog to digital, making the RME an unnecessary part of your signal chain.

If I've got you or your needs or the functioning of the RME wrong here, forgive me, but it just seems like you might have bought a bit of unnecessary equipment.

Then again, you appear to have gotten a great price on it and if I were you, assuming the thing works, I'd flip it for a profit and get one of the many great Firewire interfaces that people have recommended in this thread.
And, FWIW, I'm gonna go ahead and toss my support to the Focusrite Saffire Pro 40, which works great and has given me nothing but pristine sounding audio since day 1.
Good luck, have fun.