some would say that 'soulful' would be the opposite of 'soulless' but i don't think that is quite right
to be 'soulless' is to have soul, less
to be 'soulful' it to have soul, full
two sides of the same coin not contradictions
as an antonym for 'soulful' i would propose 'empty' or 'barren' or 'void' as opposed to the common misconception of 'soulless'
to be 'soulful' is to be 'brimming' or 'bursting' or 'overflowing' with this concept of 'soul' and 'soulless' is not necessarily an exception to possessing 'soul'