Part 3 is of Rin's data has been added.
Rin suggests the K3003's bass boost is similar to that of the EPH-100 granted both are well sealed. Rin's simulator works more accurate with IEM than even Sonove's and he made sure there was no leakage unlike SV and GoldenEars. No, the IE8 doesn't have the same boost, the IE8 will graph to about 15db on Rin's data. The bass response is actually more subbass oriented than midbass.
Rin suggests the smallest tips possible to get close to the insertion depth wanted by the manufacturer. As it's implied by the size of the housing the K3003 doesn't insert to the ear's 2nd bend or reference plane. AKG actually intended these to be about 3mm away from it but you also have to be sure to not be too shallow. The shallower the insertion from the implied depth, you get less subbass, less treble and more midhighs.
Rin suggests driving it with the least resistance possible, so use low output impedance sources. While the results aren't terrible the boost in the midhighs and treble would alter the balance already attained by the reference filter and subbass isn't needed which is also boosted by resistance.
He showed how terribly awful that bass filter is. The treble filter doesn't look too great either, I can see how some may see it as more revealing though.
AVOID blocking that vent on the nozzle, it's what keeps the bass boost in check and avoids over-pressurization of the ear-canal.