Well I bit the bullet and coughed up some $$ for 3 pairs of Polk MM651UM's, installed and tested Sunday morning (all church music of course.......).
A significant difference indeed noted, low to mid bass much improved all round. Yes I will look at a sub for those really low notes another day but these speakers have added a lot to what was missing to my ears and I'm pleased with the results. The Sony head unit is back to all zeros for the Low, Mid and High EQ settings (was 10, 3, 6 previously and lots was missing from around 300Hz downwards). Will adjust the amp gain to suit the higher efficiency of the speakers to ensure the amp doesn't become overdriven.
Thanks all for your input, much appreciated. Highly recommended mod from my experience.