@Darren Cotter
What you describe is a case of "having a good seal" vs. "having no seal". The more shallow insertion seems to give you a proper seal whereas the deeper insertion breaks the seal, resulting in almost no bass.
Usually our ear canals get narrower as they get deeper - so you could either use your IT03 the way you already do it, with the shallow insertion that however gives you a proper seal, or try to use the medium-sized tips and insert the IEMs deeper. If you get a seel with the latter, you could do it that way, or keep using the largest tips @ rather shallow insertion @ good seal.
I've re-tried the medium tips and if I push them right in and hold the IT03's for a second or two, presumably making sure they are truly snug, the sound is broadly similar. I'll do some more tip rolling, but at least I have a couple of options.
Any recommendations for after market tips or where you can purchase a set of iBasso ones?