How to clip converters on your interface?

The basics are simple. Send your mix out on 2 LL analog channels and then return them on two LL analog channels to a new track. Alternatively, set up an analog insert.

The issue if you just go analog out straight back to analog in you aren't going to clip anything. You don't want to run back into the mic pre for obvious reasons, so about your only option would be to change the reference level of the analog out to +4 and in to -10 and then adjust the output fader the RME software to achieve the amount of clipping you want coming back in.
When people do this in real life they are using an analog mastering chain (or at least partially) and using the analog gear to control the amount of clipping.
All of that said, I don't think this is worth it. Lavry and some of the other big boys design their converters to cleanly clip without issue but most mid-level stuff is going to perform much worse than software and distort in a lot of unintended ways.
 
Yeah I don't see this working out too well with just an interface. You'll have a much easier time with software. I love the T-Racks clipper. You just have to be real careful with clipping. If you're clipping the whole mix, you have to be aware of what's peaking and where. All it should really be doing is shaving a dB or two off the peaks of the drums. Anything else clipped just sounds like, well, clipping distortion, which is terrible. Maybe you're driving it too much?

FWIW the few times I've tried clipping our apogees at the studio I've sent the mix out the patch bay, into a Manley vari-mu or Smart C2, and raised the output gain of the compressor so I'm just clipping the last dB or two of snare peaks coming back in. Like egan said, you need something to raise the gain a little.
 
I'm no expert on analog clipping but I don't think anyone who does this uses their interface, especially not a mid range one. High end specialised A/D aint it?

Gclip or JST will sound better than the interface. Only use them on kick/snare/toms myself, only ever very short quick transients to get drum shells loud AF without going into red, very gently on drum bus to stop the snare and big tom hits destroying the master fader haha. Only 2 or 3 db.