Totalmix FX has loopback feature built-in which requires no physical cables and with lower latency. Your ADAT ports can then be freed up and available to other purposes. Having said that, the best and most elegant way to solve software loopbacks taking up physical IOs is to separate software IOs and Hardware IOs in the driver level. That means the input and output devices reported to the operating system are the virtual ones that do not necessarily correspond to the hardware IOs,  The patching can be done within the totalmix, so looping software A's output to software B's input only occupies available virtual IOs. However this also means we'll need an extra row in totalmix, and a much more powerful DSP in the audio interface to handle those extra mixes. If this is too much to ask, here is a much easier walk around,. Forget extra mixes which require additional processing, the audio data do not need to be passed to the hardware at all. In addition to the good old Totalmix mixer view and matrix view, add a new loopback view, where we can control which virtual output goes to which virtual input. You can't in anyway alter the audio though, just let the driver copy audio data between virtual IOs. The drawback to this method is no hardware FX can be applied (since the audio never reach the interface), and you can't mix two software outputs and send it to the third software. The driver will be responsible for looping the audio, not the audio interface, so there will be extra load on the CPU, but since the audio data are passed unaltered, the performance penalty should be fairly gentle.

Not a specific plugin but the combinator has some great presets for audio processing. Rewire is all but useless because it doesn't pass audio both ways.

Alligator, Synchronous, vocoder, scream, mclass, etc are some great modules in reason.