Impedance and Level Matching

The Octa is outputting a line level signal.  Don't know how the Reamper would take the mismatch, but I would think it was built for that kind of job, so maybe it will be fine.  There is only one way to find out ....
 
Running the line output into a preamp is an effect.  A preamp, of course, mostly raises a microphone level output to line level.  A DI does the same thing for an instrument (ie. guitar) level signal to line level.  The transformers in a preamp/DI will have an affect on the sound, esp. controlling the input and output together (if the unit has both) to saturate and distort the preamp.  The Warm Audio Tonebeast is a great example of this idea, having both knobs and choices of op amps and transformers.  As well as mic pre, DI and line in (many higher-end preamps have all these ins).  But I'm not sure you'll get anything useful from sending a line level into a cheapish IC preamp in an interface or cheap board.
 
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A DI does not amplify and change an instrument level to Line Level.  All most do is convert the unbalanced instrument input to 600 ohms balanced.    The DI would have to have an amplifier in it to amplify the instrument level input to line level.  I have about a dozen different DI units including one "active" DI and it doesn't do that.

Signal Interfacing is an often misunderstood topic, but it's really pretty simple...
 
The IEC - the body that publishes these sorts of things, makes the following recommendations:
 
For consumer gear the source impedance =< 2.2 k ohms, the input impedance >= 22 k ohms, and the nominal operating level = -10 dBV, where 0 dBV = 1.0Vrms (note it is not necessary to specify impedance!)

For professional gear the source impedance =< 1 k ohms, the input impedance >= 10 k ohms, and the nominal operating level = +4 dBu, where 0 dBu = 0.7746Vrms (again no impedance specification required.)
 
These specifications are for voltage transfer, NOT power transfer. For power transfer the source impedance and input impedance should be equal for maximum power transfer, which is still a 3 dB loss.
 
Direct boxes, re-amp boxes, etc run the gamut, and you have to read the specifications to know what you are buying.

Passive direct boxes, generally speaking, do attenuate the signal from line level (+4 dBu) to microphone level (anywhere from -30 dBu to maybe as low as -50 dBu). They do this by way of a transformer with a turns ratio that reduces the signal level.
 
Passive isolation boxes do not attenuate the signal.
 
Passive re-amp boxes (again in general) reduce the line level to something appropriate for a guitar amplifier input, typically -20 dBu to -30 dBu.
 
This is yet another opportunity for confusion, since guitar amplifiers can be described using either dBV or dBu, and the relationship between them is not obvious. It would be nice (or at least mathematically easy) if they were 14 dB apart. In fact they are only 11 dB apart, because they use different references (1V vs .775V).
 
The next obstacle is dBFS vs dBu or dBV. The later two are rms LEVEL measurements. The former is a peak level measurement. And dBFS is, at least as of now, without a fixed reference. A topic for a much longer post<G>!
 
Last bit for now, to answer the original question, is that impedance mis-matches are not as bad as they sometimes appear, you need a really big mis-match to create any kind of meaningful loss.

When people talk about impedance mismatches they often overlook the concept of the high end being effected. It is not just about level loss, it is also about high end loss. So some impedance mismatches while they may work are not good due to the high end being pulled away as well. You just have to use your ears to hear if this is happening or not.
 
It is not so usual to be patching high level line outputs back into low level mic inputs so much. You can do it for sure though. You will just need to turn the Mic Pre down gain wise in order to get the levels right. But it is a bit pointless running through a Mic Pre again.
 
Sounds like you are wanting to do loop back recording. Sonar should allow it like they do in Studio One and other DAW's too. Digitally is by the far the best way to do that. But some sound cards eg RME and EMU have complex software that goes with them that allows all sorts of routing options eg Total Mix and PatchMix.
 
You could try connecting the coaxial SPDIF OUT back to the SPDIF IN and see if you can loop back that way.