Actually what you're trying for is on-axis placement, with conventional speakers.
Ear level is misleading and a misnomer, really. The only reason people suggest ear level for front speakers is because they are normally direct firing, and the drivers will be on-axis for your ears if the tweeters are at ear level. (High frequency sounds suffer most when you get off axis.)
You can mount surrounds at ANY level and still be on-axis if you aim them. Aim them in a way that's consistent with the speaker design.
Further, given the type of stuff playing through surrounds, plus the use of high-dispersion designs for surrounds - there's quite a lot of flexibility.
IMO there are two reasons to mount surrounds above ear level ...
- It's the only way you have a chance of more or less equal volume from each ... when they are to the sides one will be much closer than the other for almost everyone, and be distractingly louder. Mounted above you the distances are less different. It's a geometry thing.
- If there's someone sitting next to you they will block the sound from that side. If the speakers are above, you can all hear.
Unless there's only 1 person listening and you can sit exactly in the middle. Which, by the way, is what you have in a production studio, which is what the link in that other thread is about ... not at all relevant to home theater.
Put them above ear level, well above ear level if need be, and aim them at the seating.