I understand Moderate and Earl's point and I agree with Lewis that this is the appropriate venue for that discussion.
On that matter, I would have no difficulty with either Peter's or Lewis's possible amendments or indeed one which is very specific in stating something such as:
"...Class B, which shall comprise Senators elected via proportional representation by means of single transferable vote."
I'd be fine with that new wording. I don't think the absolute specifics need to be in the constitution, but if the ultimate goal is to change to a PR-STV system, let's lock that in.