You already answered your own question. It is fine to use a single-ended connection, as several of us have said. (IMO: Speaker-level inputs on subs are a gimmick.)
Finally, could I run single-ended from the pre to the subs, or would that be a problem because I'm running balanced to the power amp? I understand that there'd be the possibility of noise, but I'm not certain if there's some other reason why it would not be wise to run balanced to the power amp and single-ended to the subs. If that would be OK, it would obviously open up many, many more possibilities for subs, as my searching so far has found few with XLR inputs that aren't giants.