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I've been reading that supply vents should be low wall but couldn't find it in the building code.  If  vents should be low wall in the basement does that mean return vents should be high wall or ceiling to allow proper circulation?
Also, please correct me if I am wrong:  low supply and high return would suck the cold air up to the return and push the warm air low and circulate?

Appreciate your help and the time taken to read and post a response.

There is no Code that would require low wall supply vents.  Low wall vents are usually the cause of low airflow.  I would position the return air low and leave the supply air in the ceiling.  The only time i would consider low wall is if there is a lookout/walkout condition or the home is built on a slab, but even then there is no Code.

Low wall vents cause poor airflow? Why would this be?

Always figured that since heat rises, having supply in the ceiling in the basement causes heat to remain up instead of rising from low to high.

If you measure the CFM on a ceiling vent, then add 2 elbows and run it down as a low wall you will get far less CFM.