As you might have been able to tell, I've been away for a week. I see you have already sorted the basic passage dimensions entry in Survex already. As you have observed, Aven creates blocky/square passages. Loch smooths them and looks nicer. The passage walls support in Survex is also present in builds 1.1.12 and onwards. I believe files created using Survex 1.1.14 or earlier will be readable by Loch. Something changed in 1.1.15 which makes the files incompatible with older versions of Aven and the current Loch viewer.
Some tricks to be aware of in entering passage data in Survex files. The passage walls will be created between each pair of stations in the order you enter the lines in the *data passage block. It does not pay any attention to the actual survey centreline. So if you have a junction at station 4 in a passage with stations 1-7 where your side passage starts with a station 8 then you need to put in a new '*data passage ... ' line after the LRUD data for stns 1-7, and then given LRUD data for station 4, followed by stn 8 to start a new passage. Remember that for this station 4 the LRUD data is the cross section of the new side passage at this station and not the same LRUD data for stn 4 in the main passage. Without the second '*data passage ... ' line survex will put a solid passage section from stn 7 to stn8 where no real passage exists.
I'm still getting to grips with Therion since the Mendip course, but I have learned that it will also create passage walls from the plan sketches you create, so you can create solid models from survey data which is missing the LRUD data. I've not worked out how it get the passage height in this case, as it does not seem to determine it from elevation sketches as far as I can tell.